# /=====================================================================\ #
# |  LaTeXML::MathParser                                                | #
# | Parse Math                                                          | #
# |=====================================================================| #
# | Part of LaTeXML:                                                    | #
# |  Public domain software, produced as part of work done by the       | #
# |  United States Government & not subject to copyright in the US.     | #
# |---------------------------------------------------------------------| #
# | Bruce Miller <bruce.miller@nist.gov>                        #_#     | #
# | http://dlmf.nist.gov/LaTeXML/                              (o o)    | #
# \=========================================================ooo==U==ooo=/ #
# ================================================================================
# LaTeXML::MathParser  Math Parser for LaTeXML using Parse::RecDescent.
# Parse the intermediate representation generated by the TeX processor.
# ================================================================================
package LaTeXML::MathParser;
use strict;
use warnings;
use Parse::RecDescent;
use LaTeXML::Global;
use LaTeXML::Common::Object;
use LaTeXML::Common::Error;
use LaTeXML::Core::Token;
use LaTeXML::Common::Font;
use LaTeXML::Common::XML;
use List::Util qw(min max);
use base (qw(Exporter));

our @EXPORT_OK = (qw(&Lookup &New &Absent &Apply &ApplyNary &recApply &CatSymbols
    &Annotate &InvisibleTimes &InvisibleComma
    &NewFormulae &NewFormula &NewList
    &ApplyDelimited &NewScript &DecorateOperator &InterpretDelimited &NewEvalAt
    &LeftRec
    &Arg &MaybeFunction
    &SawNotation &IsNotationAllowed
    &isMatchingClose &Fence));
our %EXPORT_TAGS = (constructors
    => [qw(&Lookup &New &Absent &Apply &ApplyNary &recApply &CatSymbols
      &Annotate &InvisibleTimes &InvisibleComma
      &NewFormulae &NewFormula &NewList
      &ApplyDelimited &NewScript &DecorateOperator &InterpretDelimited &NewEvalAt
      &LeftRec
      &Arg &MaybeFunction
      &SawNotation &IsNotationAllowed
      &isMatchingClose &Fence)]);

# ================================================================================
sub new {
  my ($class, %options) = @_;
  require LaTeXML::MathGrammar;

  my $internalparser = LaTeXML::MathGrammar->new();
  Fatal("expected", "MathGrammar", undef,
    "Compilation of Math Parser grammar failed") unless $internalparser;

  my $self = bless { internalparser => $internalparser }, $class;
  return $self; }

sub parseMath {
  my ($self, $document, %options) = @_;
  local $LaTeXML::MathParser::DOCUMENT = $document;
  $self->clear;    # Not reentrant!
  $self->cleanupScripts($document);
  if (my @math = $document->findnodes('descendant-or-self::ltx:XMath[not(ancestor::ltx:XMath)]')) {
    NoteBegin("Math Parsing"); NoteProgress(scalar(@math) . " formulae ...");
#### SEGFAULT TEST
####    $document->doctest("before parse",1);
    foreach my $math (@math) {
      $self->parse($math, $document); }

    NoteProgress("\nMath parsing succeeded:"
        . join('', map { "\n   $_: "
            . colorizeString($$self{passed}{$_} . "/" . ($$self{passed}{$_} + $$self{failed}{$_}), ($$self{failed}{$_} == 0 ? 'success' : 'warning')) }
          grep { $$self{passed}{$_} + $$self{failed}{$_} }
          keys %{ $$self{passed} }) . "\n");
    if (my @unk = keys %{ $$self{unknowns} }) {
      NoteProgress("Symbols assumed as simple identifiers (with # of occurences):\n   "
          . join(', ', map { "'" . colorizeString("$_", 'warning') . "' ($$self{unknowns}{$_})" } sort @unk) . "\n");
      if (!$STATE->lookupValue('MATHPARSER_SPECULATE')) {
        NoteProgress("Set MATHPARSER_SPECULATE to speculate on possible notations.\n"); } }
    if (my @funcs = keys %{ $$self{maybe_functions} }) {
      NoteProgress("Possibly used as functions?\n  "
          . join(', ', map { "'$_' ($$self{maybe_functions}{$_}/$$self{unknowns}{$_} usages)" }
            sort @funcs) . "\n"); }
#### SEGFAULT TEST
####    $document->doctest("IN scope",1);
    NoteEnd("Math Parsing"); }
#### SEGFAULT TEST
####    $document->doctest("OUT of scope",1);
  return $document; }

# This is a rather peculiar cleanup that needs to be done to manage ids & idrefs
# Before parsing, sub/superscripts are represented by an operator-less XMApp with the script
# itself as the only child. Ideally, upon parsing these get merged, combined and disappear into
# proper XMApp of an appropriate operator on the base and scripts.  Unless there is a parse
# failure, in which case they remain.
# The problem comes from various XMDual constructs where an XMRef refers to the script XMApp.
# It can occur that one branch parses and the other fails: This can leave a reference to
# the script XMApp which no longer exists!
# To solve this, we find & replace all references to such script XMApps by an explicit XMApp
# with the XMRef refering to the script itself, not the XMApp. (make sense?)
sub cleanupScripts {
  my ($self, $document) = @_;
  foreach my $app ($document->findnodes(
      'descendant-or-self::ltx:XMApp[@xml:id and contains(@role,"SCRIPT")]')) {
    my $role  = $app->getAttribute('role');
    my $appid = $app->getAttribute('xml:id');
    if ($role =~ /^(?:PRE|POST|FLOAT)(:?SUB|SUPER)SCRIPT$/) {
      my @refs = $document->findnodes("descendant-or-self::ltx:XMRef[\@idref = '$appid']");
      if (scalar(@refs)) {
#        print STDERR "\nREPLACING SCRIPT REF: found " . scalar(@refs) . " references to " . ToString($app) . "\n";
        my $script = $app->firstChild;
        my ($scriptref) = LaTeXML::Package::createXMRefs($document, $script);
        $document->unRecordID($appid);    # no longer refers to the app
        $app->removeAttribute('xml:id');
        # Copy all attributes, EXCEPT xml:id
        my %attr = map { (getQName($_) => $_->getValue) }
          grep { $_->nodeType == XML_ATTRIBUTE_NODE } $app->attributes;
        # Now, replace each ref to the script application by an application to a ref to the script.
        foreach my $ref (@refs) {
          $document->replaceTree(['ltx:XMApp', {%attr}, $scriptref], $ref); }
      } } }
  return; }

sub getQName {
  my ($node) = @_;
  if (ref $node eq 'ARRAY') {
    return $$node[0]; }
  else {
    return $LaTeXML::MathParser::DOCUMENT->getModel->getNodeQName($node); } }

sub realizeXMNode {
  my ($node) = @_;
  my $doc = $LaTeXML::MathParser::DOCUMENT;
  my $idref;
  if (!defined $node) {
    return; }
  elsif (ref $node eq 'ARRAY') {
    $idref = $$node[1]{idref} if $$node[0] eq 'ltx:XMRef'; }
  elsif (ref $node eq 'XML::LibXML::Element') {
    $idref = $node->getAttribute('idref')
      if $doc->getModel->getNodeQName($node) eq 'ltx:XMRef'; }
  if ($idref) {
    # Can it happen that $realnode is, itself, an XMRef? Then we should recurse!
    if (my $realnode = $doc->lookupID($idref)) {
      return $realnode; }
    else {
      Error("expected", 'id', undef, "Cannot find a node with xml:id='$idref'",
        ($LaTeXML::MathParser::IDREFS{$idref}
          ? "Previously bound to " . ToString($LaTeXML::MathParser::IDREFS{$idref})
          : ()));
      return ['ltx:ERROR', {}, "Missing XMRef idref=$idref"]; } }
  else {
    return $node; } }

# ================================================================================
sub clear {
  my ($self) = @_;
  $$self{passed} = { 'ltx:XMath' => 0, 'ltx:XMArg' => 0, 'ltx:XMWrap' => 0 };
  $$self{failed} = { 'ltx:XMath' => 0, 'ltx:XMArg' => 0, 'ltx:XMWrap' => 0 };
  $$self{unknowns}        = {};
  $$self{maybe_functions} = {};
  $$self{n_parsed}        = 0;
  return; }

our %EXCLUDED_PRETTYNAME_ATTRIBUTES = (fontsize => 1, opacity => 1);

sub token_prettyname {
  my ($node) = @_;
  my $name = $node->getAttribute('name');
  if (defined $name) { }
  elsif ($name = $node->textContent) {
    my $font = $LaTeXML::MathParser::DOCUMENT->getNodeFont($node);
    my %attr = $font->relativeTo(LaTeXML::Common::Font->textDefault);
    my $desc = join(' ', map { ToString($attr{$_}{value}) }
        (grep { !$EXCLUDED_PRETTYNAME_ATTRIBUTES{$_} } (sort keys %attr)));
    $name .= "{$desc}" if $desc; }
  else {
    $name = Stringify($node); }    # what else ????
  return $name; }

sub note_unknown {
  my ($self, $node) = @_;
  my $name = token_prettyname($node);
  $$self{unknowns}{$name}++;
  return; }

# debugging utility, should be somewhere handy.
sub printNode {
  my ($node) = @_;
  if (ref $node eq 'ARRAY') {
    my ($tag, $attr, @children) = @$node;
    my @keys = sort keys %$attr;
    return "<$tag"
      . (@keys ? ' ' . join(' ', map { "$_='$$attr{$_}'" } @keys) : '')
      . (@children
      ? ">\n" . join('', map { printNode($_) } @children) . "</$tag>"
      : '/>')
      . "\n"; }
  else {
    return ToString($node); } }

# ================================================================================
# Some more XML utilities, but math specific (?)

# Get the Token's  meaning, else name, else content, else role
sub getTokenMeaning {
  my ($node) = @_;
  my $x;
  $node = realizeXMNode($node);
  return (defined($x = p_getAttribute($node, 'meaning')) ? $x
    : (defined($x = p_getAttribute($node, 'name')) ? $x
      : (($x = (ref $node eq 'ARRAY' ? '' : $node->textContent)) ne '' ? $x
        : (defined($x = p_getAttribute($node, 'role')) ? $x
          : undef)))); }

sub node_location {
  my ($node) = @_;
  my $n = $node;
  while ($n && (ref $n !~ /^XML::LibXML::Document/)    # Sometimes DocuementFragment ???
    && !$n->getAttribute('refnum') && !$n->getAttribute('labels')) {
    $n = $n->parentNode; }
  if ($n && (ref $n !~ /^XML::LibXML::Document/)) {
    my ($r, $l) = ($n->getAttribute('refnum'), $n->getAttribute('labels'));
    return ($r && $l ? "$r ($l)" : $r || $l); }
  else {
    return 'Unknown'; } }

#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# Parser
#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# Top-level per-formula parse.
# We do a depth-first traversal of the content of the XMath element,
# since various sub-elements (XMArg & XMWrap) act as containers of
# nominally complete subexpressions.
# We do these first for two reasons.
# Firstly, since after parsing, the parent will be rebuilt from the result,
# we lose the node "identity"; ie. we can't find the child to replace it!
# Secondly, in principle (although this isn't used yet), parsing the
# child could reveal something interesting about it; say, it's effective role.
# Then, this information could be used when parsing the parent.
# In fact, this could work the other way too; parsing the parent could tell
# us something about what the child must be....
sub parse {
  my ($self, $xnode, $document) = @_;
  local $LaTeXML::MathParser::STRICT      = 1;
  local $LaTeXML::MathParser::WARNED      = 0;
  local $LaTeXML::MathParser::XNODE       = $xnode;
  local $LaTeXML::MathParser::PUNCTUATION = {};
  local $LaTeXML::MathParser::LOSTNODES   = {};
  local %LaTeXML::MathParser::IDREFS      = ();
  # This bit for debugging....
  foreach my $n ($document->findnodes("descendant-or-self::*[\@xml:id]", $xnode)) {
    my $id = $n->getAttribute('xml:id');
    $LaTeXML::MathParser::IDREFS{$id} = $n; }
  if (my $result = $self->parse_rec($xnode, 'Anything,', $document)) {
    # Add text representation to the containing Math element.
    my $p = $xnode->parentNode;
    # This is a VERY screwy situation? How can the parent be a document fragment??
    # This has got to be a LibXML bug???
    if ($p->nodeType == XML_DOCUMENT_FRAG_NODE) {
      my @n = $p->childNodes;
      if (scalar(@n) == 1) {
        $p = $n[0]; }
      else {
        Fatal('malformed', '<XMath>', $xnode, "XMath node has DOCUMENT_FRAGMENT for parent!"); } }
    # HACK: replace XMRef's to stray trailing punctution
    foreach my $id (keys %$LaTeXML::MathParser::PUNCTUATION) {
      my $r = $$LaTeXML::MathParser::PUNCTUATION{$id}->cloneNode;
      $r->removeAttribute('xml:id');
      foreach my $n ($document->findnodes("descendant-or-self::ltx:XMRef[\@idref='$id']", $p)) {
        $document->replaceTree($r, $n); } }
    foreach my $id (keys %$LaTeXML::MathParser::LOSTNODES) {
      my $repid = $$LaTeXML::MathParser::LOSTNODES{$id};
      # but the replacement my have been replaced as well!
      while (my $reprepid = $$LaTeXML::MathParser::LOSTNODES{$repid}) {
        $repid = $reprepid; }
      foreach my $n ($document->findnodes("descendant-or-self::ltx:XMRef[\@idref='$id']", $p)) {
        $document->setAttribute($n, idref => $repid); } }

    $p->setAttribute('text', text_form($result)); }
  return; }

my %TAG_FEEDBACK = ('ltx:XMArg' => 'a', 'ltx:XMWrap' => 'w');    # [CONSTANT]
# Recursively parse a node with some internal structure
# by first parsing any structured children, then it's content.
sub parse_rec {
  my ($self, $node, $rule, $document) = @_;
  $self->parse_children($node, $document);
  # This will only handle 1 layer nesting (successfully?)
  # Note that this would have been found by the top level xpath,
  # but we've got to worry about node identity: the parent is being rebuilt
  foreach my $nested ($document->findnodes('descendant::ltx:XMath', $node)) {
    $self->parse($nested, $document); }
  my $tag = getQName($node);
  if (my $requested_rule = $node->getAttribute('rule')) {
    $rule = $requested_rule; }
  if ($rule eq 'kludge') {
    $self->parse_kludge($node, $document);
    return; }
  elsif (my $result = $self->parse_single($node, $document, $rule)) {
    $$self{passed}{$tag}++;
    if ($tag eq 'ltx:XMath') {    # Replace the content of XMath with parsed result
      NoteProgress('[' . ++$$self{n_parsed} . ']');
      map { $document->unRecordNodeIDs($_) } element_nodes($node);
      # unbindNode followed by (append|replace)Tree (which removes ID's) should be safe
      map { $_->unbindNode() } $node->childNodes;
      $document->appendTree($node, $result);
      $result = [element_nodes($node)]->[0]; }
    else {                        # Replace the whole node for XMArg, XMWrap; preserve some attributes
      NoteProgressDetailed($TAG_FEEDBACK{$tag} || '.');
      # Copy all attributes
      my $resultid = p_getAttribute($result, 'xml:id');
      my %attr = map { (getQName($_) => $_->getValue) }
        grep { $_->nodeType == XML_ATTRIBUTE_NODE } $node->attributes;
      # add to result, even allowing modification of xml node, since we're committed.
      # [Annotate converts node to array which messes up clearing the id!]
      my $isarr = ref $result eq 'ARRAY';
      my $rtag = ($isarr ? $$result[0] : $document->getNodeQName($result));
      # Make sure font is "Appropriate", if we're creating a new token (yuck)
      if ($isarr && $attr{_font} && ($rtag eq 'ltx:XMTok')) {
        my $content = join('', @$result[2 .. $#$result]);
        if ((!defined $content) || ($content eq '')) {
          delete $attr{_font}; }    # No font needed
        elsif (my $font = $document->decodeFont($attr{_font})) {
          delete $attr{_font};
          $attr{font} = $font->specialize($content); } }
      else {
        delete $attr{_font}; }
      foreach my $key (keys %attr) {
        next unless ($key =~ /^_/) || $document->canHaveAttribute($rtag, $key);
        my $value = $attr{$key};
        if ($key eq 'xml:id') {     # Since we're moving the id...bookkeeping
          $document->unRecordID($value);
          $node->removeAttribute('xml:id'); }
        if ($isarr) { $$result[1]{$key} = $value; }
        else { $document->setAttribute($result, $key => $value); } }
      $result = $document->replaceTree($result, $node);
      my $newid = $attr{'xml:id'};
      # Danger: the above code replaced the id on the parsed result with the one from XMArg,..
      # If there are any references to $resultid, we need to point them to $newid!
      if ($resultid && $newid && ($resultid ne $newid)) {
        foreach my $ref ($document->findnodes("//*[\@idref='$resultid']")) {
          $ref->setAttribute(idref => $newid); } }
    }
    return $result; }
  else {
    $self->parse_kludge($node, $document);
    if ($tag eq 'ltx:XMath') {
      NoteProgress('[F' . ++$$self{n_parsed} . ']'); }
    elsif ($tag eq 'ltx:XMArg') {
      NoteProgressDetailed('-a'); }
    $$self{failed}{$tag}++;
    return; } }

# Depth first parsing of XMArg nodes.
sub parse_children {
  my ($self, $node, $document) = @_;
  foreach my $child (element_nodes($node)) {
    my $tag = getQName($child);
    if ($tag eq 'ltx:XMArg') {
      $self->parse_rec($child, 'Anything', $document); }
    elsif ($tag eq 'ltx:XMWrap') {
      local $LaTeXML::MathParser::STRICT = 0;
      $self->parse_rec($child, 'Anything', $document); }
### A nice evolution would be to use the Kludge parser for
### the presentation form in XMDual
### This would avoid silly "parses" of non-semantic stuff; eg assuming times between tokens!
### However, it needs some experimentation to match DLMF's enhancements
####      $self->parse_children($child,$document);
####      $self->parse_kludge($child,$document); }
    elsif ($tag =~ /^ltx:(XMApp|XMArray|XMRow|XMCell)$/) {
      $self->parse_children($child, $document); }
    elsif ($tag eq 'ltx:XMDual') {
      $self->parse_children($child, $document); }
    ### Some day this may be needed? but not yet...
   #     elsif ($tag eq 'ltx:XMRef') {
   #       # We might be referencing something outside the current tree.
   #       # but we'd like it's contents to be parsed before we start using it at this level.
   #       # NOTE: Do we have to worry about things being parsed twice? (ie. when the other tree's done)
   #       # if so, we probably want to mark things as having already been parsed.
   #       my $refnode = realizeXMNode($child);
   #       if(getQName($refnode) ne 'ltx:XMTok'){ # Anything referenced with structure...
   #         print STDERR "PRE-PARSING Ref'd node: ".ToString($child)." == ".ToString($refnode). "\n";
   # ##        $self->parse_rec($refnode,'Anything',$document);
   #         $self->parse_children($refnode,$document);
   #         print STDERR "NOW REFERENCES: ".ToString(realizeXMNode($child))."\n";
   #  } }
  }
  return; }

my $HINT_PUNCT_THRESHOLD = 10.0;    # \quad or bigger becomes punctuation ? [CONSTANT]

sub filter_hints {
  my ($self, $document, @nodes) = @_;
  my @prefiltered      = ();
  my $prev             = undef;
  my $pending_comments = '';
  my $pending_space    = 0.0;
  # Filter the nodes, watching for XMHint's and Comments.
  foreach my $node (@nodes) {
    my $type = $node->nodeType;
    if ($type == XML_TEXT_NODE) {    # text? better be (ignorable) whitespace!
      my $string = $node->textContent;
      if ($string =~ /\S/) {
        Warn('unexpected', 'text', $node, "Unexpected text in Math tree"); } }
    elsif ($type == XML_COMMENT_NODE) {    # Comment!
      my $comment = $node->getData;
      if ($prev) {                         # Append to previous element's comments
        my $c = $prev->getAttribute('_comment');
        $prev->setAttribute(_comment => ($c ? $c . "\n" . $comment : $comment)); }
      else {                               # Or save for first
        $pending_comments = ($pending_comments ? $pending_comments . "\n" . $comment : $comment); } }
    elsif ($type != XML_ELEMENT_NODE) {
      Warn('unexpected', 'node', $node, "Unexpected item in Math tree"); }
    elsif (getQName($node) eq 'ltx:XMHint') {    # If a Hint node?
      if (my $width = $node->getAttribute('width')) {
        if (my $pts = getXMHintSpacing($width)) {
          if ($prev) {
            my $s = $prev->getAttribute('_space') || 0.0;
            $prev->setAttribute(_space => $s + $pts); }
          else {
            $pending_space += $pts; } } } }
    else {                                       # Some other element.
      if ($pending_comments) {
        $node->setAttribute(_pre_comment => $pending_comments);
        $pending_comments = ''; }
      if ($pending_space) {
        $node->setAttribute(lpadding =>
            LaTeXML::Common::Dimension::attributeformat($pending_space * 65536));
        $pending_space = 0.0; }
      push(@prefiltered, $node); $prev = $node; }    # Keep it.
  }
  my @filtered = ();
  # Filter through the pre-filtered nodes looking for large rpadding.
  foreach my $node (@prefiltered) {
    push(@filtered, $node);
    if (my $s = $node->getAttribute('_space')) {
      $node->removeAttribute('_space');
      if (($s >= $HINT_PUNCT_THRESHOLD) && (($node->getAttribute('role') || '') ne 'PUNCT')) {
        # Create a new Punctuation node (XMTok) from the wide space
        # I'm leary that this is a safe way to create an XML node that's NOT in the tree, but...
        my $p = $node->parentNode;
        #        my $punct = $document->openElementAt($p,'ltx:XMTok',role=>'PUNCT',rpadding=>$s.'pt');
        my $punct = $document->openElementAt($p, 'ltx:XMTok', role => 'PUNCT',
          name => ('q' x int($s / 10)) . 'uad');
        $punct->appendText(spacingToString($s));
        $p->removeChild($punct);    # But don't actually leave it in the tree!!!!
        push(@filtered, $punct); }
      else {
        $node->setAttribute(rpadding =>
            LaTeXML::Common::Dimension::attributeformat($s * 65536)); } } }
  return @filtered; }

# Given a width attribute on an XMHint, return the pts, if any
sub getXMHintSpacing {
  my ($width) = @_;
  if ($width && ($width =~ /^$LaTeXML::Common::Glue::GLUE_re$/)) {
    return ($2 eq 'mu' ? $1 / 1.8 : $1); }
  else {
    return 0; } }

# We've pretty much builtin the assumption that the target XML is "As If" 10 pts,
# so we'll assume that 1em is 10 pts.
my $POINTS_PER_EM = 10.0;    # [CONSTANT]
# Convert spacing, given as a number of points, to a string of appropriate spacing chars
sub spacingToString {
  my ($points) = @_;
  my $spacing  = '';
  my $ems      = $points / $POINTS_PER_EM;
  my $n        = int($ems);
  if ($n > 0)       { $spacing .= ("\x{2003}" x $n); $ems -= $n; }
  if ($ems > 0.500) { $spacing .= "\x{2002}";        $ems -= 0.500; }
  if ($ems > 0.333) { $spacing .= "\x{2003}";        $ems -= 0.333; }
  if ($ems > 0.250) { $spacing .= "\x{2005}";        $ems -= 0.250; }
  if ($ems > 0.166) { $spacing .= "\x{2006}";        $ems -= 0.166; }
  return $spacing; }

#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# Low-Level hack parsing when "real" parsing fails;
# Two issues cause generated Presentation MathML to be really bad:
# (1) not having mrow/mfenced structures wrapping OPEN...CLOSE sequences
#     throws off MathML's stretchiness treatment of the fences
#     (they're all the same size; big)
# (2) un-attached sub/superscripts won't position correctly,
#     unless they're attached to something plausible.
# NOTE: we should be able to optionally switch this off.
# Especially, when we want to try alternative parse strategies.
sub parse_kludge {
  my ($self, $mathnode, $document) = @_;
  my @nodes = $self->filter_hints($document, $mathnode->childNodes);
  # the 1st array in stack accumlates the nodes within the current fenced row.
  # When there's only a single array, it's single entry will be the complete row.
  my @stack = ([], []);
  my @pairs = map { [$_, $self->getGrammaticalRole($_)] } @nodes;
  while (scalar(@pairs) || (scalar(@stack) > 1)) {
    my $pair = shift(@pairs);
    my $role = ($pair ? $$pair[1] : 'CLOSE');
    if ($role eq 'OPEN') {
      unshift(@stack, [$pair]); }    # Start new fenced row;
    elsif ($role eq 'CLOSE') {       # Close the current row
      my $row = shift(@stack);       # get the current list of items
      push(@$row, $pair) if $pair;   # Put the close (if any) into it
      my @kludged = $self->parse_kludgeScripts_rec(@$row);    # handle scripts
                                                              # wrap, if needed.
      $row = [(scalar(@kludged) > 1 ? ['ltx:XMWrap', {}, @kludged] : $kludged[0]), 'FENCED'];
      push(@{ $stack[0] }, $row); }    # and put this constructed row at end of containing row.
    else {
      push(@{ $stack[0] }, $pair); } }    # Otherwise, just put this item into current row.

  # If we got to here, remove the nodes and replace them by the kludged structure.
  map { $document->unRecordNodeIDs($_) } element_nodes($mathnode);
  # unbindNode followed by (append|replace)Tree (which removes ID's) should be safe
  map { $_->unbindNode() } $mathnode->childNodes;
  # We're hoping for a single list on the stack,
  # But extra CLOSEs will leave extra junk behind, so process all the stacked lists.
  my @replacements = ();
  foreach my $pair (@{ $stack[0] }) {
    my $kludge = $$pair[0];
    push(@replacements, (ref $kludge eq 'ARRAY') && ($$kludge[0] eq 'ltx:XMWrap')
      ? @$kludge[2 .. $#$kludge] : ($kludge)); }
  $document->appendTree($mathnode, @replacements);
  return; }

sub parse_kludgeScripts_rec {
  my ($self, $x, $y, @more) = @_;
  my @acc = ();
  while (defined $y) {
    if ($$x[1] =~ /^FLOAT(SUB|SUPER)SCRIPT$/) {    # a Floating script? maybe pre-script
      if ($$y[1] =~ /^POST(SUB|SUPER)SCRIPT$/) {    # followed by another script?
        if (@more) {                                # followed by a something? => Combined pre sub & super
          my ($base, @rest) = $self->parse_kludgeScripts_rec(@more);
          return (@acc, NewScript(NewScript($base, $$y[0], 'pre'), $$x[0], 'pre'), @rest); }
        else {                                      # else floating sub & super
          return (@acc, NewScript(NewScript(Absent(), $$y[0], 'post'), $$x[0], 'post')); } }
      else {                                        # else just prescript on whatever follows
        my ($base, @rest) = $self->parse_kludgeScripts_rec($y, @more);
        return (@acc, NewScript($base, $$x[0], 'pre'), @rest); } }
    elsif ($$y[1] =~ /^POST(SUB|SUPER)SCRIPT$/) {
      # or a postscript is applied to the preceding thing.
      return (@acc, $self->parse_kludgeScripts_rec([NewScript($$x[0], $$y[0]), ''], @more)); }
    else {                                          # else skip over a and continue
      push(@acc, $$x[0]);
      ($x, $y, @more) = ($y, @more); } }
  return (@acc, $$x[0]); }

# OBSOLETE EXPLORATORY CODE; but there are some interesting ideas....
# sub parse_kludge {
#   my($self,$mathnode,$document)=@_;
#   my @nodes = $self->filter_hints($document,$mathnode->childNodes);
#   map { $mathnode->removeChild($_) } @nodes;
#   my @result=();
#   while(@nodes){
#     @nodes = $self->parse_kludge_rec(@nodes);
#     push(@result,shift(@nodes)); }
#   $document->appendTree($mathnode,@result); }

# # Kludge Parse the next thing, and then add any following scripts to it.
# sub parse_kludge_rec {
#   my($self,@more)=@_;
#   my($item,$open,$close,$seps,$arg);
#   ($item,@more) = $self->parse_kludge_reca(@more);
#   while(@more && (($self->getGrammaticalRole($more[0])||'') =~ /^POST(SUB|SUPER)SCRIPT$/)){
#     $item = NewScript($item,shift(@more)); }
#   if(@more && (($self->getGrammaticalRole($more[0])||'') eq 'APPLYOP')){
#     shift(@more);
#     if(@more && (($self->getGrammaticalRole($more[0])||'') eq 'OPEN')){
#       ($open,$close,$seps,$arg,@more)=$self->parse_kludge_fence(@more);
#       $item = Apply(Annotate($item,argopen=>$open, argclose=>$close, separators=>$seps),@$arg); }
#     else {
#       ($arg,@more)=$self->parse_kludge_rec(@more);
#       $item = Apply($item,$arg); }}
#   ($item,@more); }

# sub parse_kludge_reca {
#   my($self,$next,@more)=@_;
#   my $role = $self->getGrammaticalRole($next);
#   if($role =~ /^FLOAT(SUB|SUPER)SCRIPT$/){
#     my($base,@rest) = $self->parse_kludge_rec(@more);
#     (NewScript($base,$next),@rest); }
#   elsif($role eq 'OPEN'){
#     my($open,$close,$seps,$list,@more)=$self->parse_kludge_fence($next,@more);
#     (Apply(Annotate(New(undef,undef,role=>'FENCED'),
#     argopen=>$open, argclose=>$close, separators=>$seps),@$list), @more); }
#   else {
#     ($next,@more); }}

# sub parse_kludge_fence {
#   my($self,$next,@more)=@_;
#   my($open,$close,$punct,$r,$item,@list)=($next,undef,'',undef);
#   while(@more){
#     my @i=();
#     while(@more && (($r=($self->getGrammaticalRole($more[0])||'')) !~ /^(CLOSE|PUNCT)$/)){
#       ($item,@more)=$self->parse_kludge_rec(@more);
#       push(@i,$item); }
#     push(@list,(scalar(@i > 1) ? ['ltx:XMWrap',{},@i] : $i[0]));
#     if($r eq 'CLOSE'){
#       $close=shift(@more); last; }
#     else {
#       $punct .= ($punct ? ' ':''). p_getValue(shift(@more)); }} # Delimited by SINGLE SPACE!
#   ($open,$close,$punct,[@list],@more); }

#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# Low-level Parser: parse a single expression
#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# Convert to textual form for processing by MathGrammar
sub parse_single {
  my ($self, $mathnode, $document, $rule) = @_;
  my @nodes = $self->filter_hints($document, $mathnode->childNodes);

  my ($punct, $result, $unparsed);
  my @punct = ();
  # Extract trailing punctuation, if rule allows it.
  if ($rule =~ s/,$//) {
    # Collect ALL trailing PUNCT|PERIOD's...
    my ($x, $r);
    while (($x = $nodes[-1]) && ($x = realizeXMNode($x)) && (getQName($x) eq 'ltx:XMTok')
      && ($r = $x->getAttribute('role')) && (($r eq 'PUNCT') || ($r eq 'PERIOD'))) {
      my $p = pop(@nodes);
      # Special case hackery, in case this thing is XMRef'd!!!
      # We could just stick it on the end of the presentation,
      # but it doesn't belong in the content at all!?!?
      if (my $id = $p->getAttribute('xml:id')) {
        $$LaTeXML::MathParser::PUNCTUATION{$id} = $p; }
      unshift(@punct, $p); }
  }

  if ($LaTeXML::MathParser::DEBUG) {
    $::RD_TRACE = 1;    # Turn on MathGrammar tracing
                        #    my $box = $document->getNodeBox($LaTeXML::MathParser::XNODE);
    my $box = $document->getNodeBox($mathnode);
    print STDERR "\n" . ('=' x 60) .
      "\nParsing formula \"" . ToString($box) . "\""
      . "\n from " . $box->getLocator
      . "\n == \"" . join(' ', map { node_string($_, $document) } @nodes) . "\""
      . "\n == " . ToString($mathnode)
      . "\n"; }

  if (scalar(@nodes) < 2) {    # Too few nodes? What's to parse?
    $result = $nodes[0] || Absent(); }
  else {
    # Now do the actual parse.
    ($result, $unparsed) = $self->parse_internal($rule, @nodes); }

  # Failure? No result or uparsed lexemes remain.
  # NOTE: Should do script hack??
  if ((!defined $result) || $unparsed) {
    $self->failureReport($document, $mathnode, $rule, $unparsed, @nodes);
    return; }
  # Success!
  else {
    if (@punct) {    # create a trivial XMDual to treat the punctuation as presentation
      $result = ['ltx:XMDual', {},
        LaTeXML::Package::createXMRefs($document, $result),
        ['ltx:XMWrap', {}, $result, @punct]]; }    # or perhaps: Apply, punctuated???
    if ($LaTeXML::MathParser::DEBUG) {
      print STDERR "\n=>" . printNode($result) . "\n" . ('=' x 60) . "\n"; }
    return $result; } }

sub parse_internal {
  my ($self, $rule, @nodes) = @_;
  #------------
  # Generate a textual token for each node; The parser operates on this encoded string.
  local $LaTeXML::MathParser::LEXEMES = {};
  my $i       = 0;
  my $lexemes = '';
  foreach my $node (@nodes) {
    my $role = $self->getGrammaticalRole($node);
    my $text = getTokenMeaning($node);
    $text = 'Unknown' unless defined $text;
    my $lexeme = $role . ":" . $text . ":" . ++$i;
    $lexeme =~ s/\s//g;
    $$LaTeXML::MathParser::LEXEMES{$lexeme} = $node;
    $lexemes .= ' ' . $lexeme; }

  #------------
  # apply the parser to the textified sequence.
  local $LaTeXML::MathParser::PARSER               = $self;
  local %LaTeXML::MathParser::SEEN_NOTATIONS       = ();
  local %LaTeXML::MathParser::DISALLOWED_NOTATIONS = ();
  local $LaTeXML::MathParser::MAX_ABS_DEPTH        = 1;
  # "Speculative" mode is disabled by default (set MATHPARSER_SPECULATE).
  # It causes the parser to explore possible undeclared notatations,
  # like possible functions (the only one, so far)
  # These extra grammar rules can be costly in corner cases.
  # This should be disabled when speed (but feedback) is essential
  if ($STATE->lookupValue('MATHPARSER_SPECULATE')) {
    $LaTeXML::MathParser::DISALLOWED_NOTATIONS{MaybeFunctions} = 0; }
  else {
    $LaTeXML::MathParser::DISALLOWED_NOTATIONS{MaybeFunctions} = 1; }
  my $unparsed = $lexemes;
  my $result   = $$self{internalparser}->$rule(\$unparsed);
  if (((!defined $result) || $unparsed)    # If parsing Failed
    && $LaTeXML::MathParser::SEEN_NOTATIONS{QM}) {    # & Saw some QM stuff.
    $LaTeXML::MathParser::DISALLOWED_NOTATIONS{QM} = 1;    # Retry w/o QM notations
    $unparsed = $lexemes;
    $result = $$self{internalparser}->$rule(\$unparsed); }
  while (((!defined $result) || $unparsed)                 # If parsing Failed
    && ($LaTeXML::MathParser::SEEN_NOTATIONS{AbsFail})     # & Attempted deeper abs nesting?
    && ($LaTeXML::MathParser::MAX_ABS_DEPTH < 3)) {        # & Not ridiculously deep
    delete $LaTeXML::MathParser::SEEN_NOTATIONS{AbsFail};
    ++$LaTeXML::MathParser::MAX_ABS_DEPTH;                 # Try deeper.
    $unparsed = $lexemes;
    $result   = $$self{internalparser}->$rule(\$unparsed); }

  # If still failed, try other strategies?

  return ($result, $unparsed); }

sub getGrammaticalRole {
  my ($self, $node) = @_;
  $node = realizeXMNode($node);
  #  my $role = $node->getAttribute('role');
  my $role = p_getAttribute($node, 'role');
  if (!defined $role) {
    my $tag = getQName($node);
    if ($tag eq 'ltx:XMTok') {
      $role = 'UNKNOWN'; }
    elsif ($tag eq 'ltx:XMDual') {
      $role = $LaTeXML::MathParser::DOCUMENT->getFirstChildElement($node)->getAttribute('role'); }
    $role = 'ATOM' unless defined $role; }
  $self->note_unknown($node) if ($role eq 'UNKNOWN') && $LaTeXML::MathParser::STRICT;
  return $role; }

# How many tokens before & after the failure point to report in the Warning message.
my $FAILURE_PRETOKENS  = 3;    # [CONSTANT]
my $FAILURE_POSTTOKENS = 1;    # [CONSTANT]

sub failureReport {
  my ($self, $document, $mathnode, $rule, $unparsed, @nodes) = @_;
  if ($LaTeXML::MathParser::STRICT || (($STATE->lookupValue('VERBOSITY') || 0) > 1)) {
    my $loc = "";
    # If we haven't already done it for this formula, show the original TeX.
    if (!$LaTeXML::MathParser::WARNED) {
      $LaTeXML::MathParser::WARNED = 1;
      my $box = $document->getNodeBox($LaTeXML::MathParser::XNODE);
      $loc = "In \"" . UnTeX($box) . "\""; }
    $unparsed =~ s/^\s*//;
    my @rest = split(/ /, $unparsed);
    my $pos = scalar(@nodes) - scalar(@rest);
    # Break up the input at the point where the parse failed.
    my $max = 50;
    my $parsed = join(' ', ($pos > $max ? ('...') : ()),
      (map { node_string($_, $document) } @nodes[max(0, $pos - 50) .. $pos - 1]));
    my $toparse = join(' ',
      (map { node_string($_, $document) } @nodes[$pos .. min($pos + 50, $#nodes)]),
      ($#nodes > $pos + 50 ? ('...') : ()));
    my $parsefail
      = join('.', map { $self->getGrammaticalRole($_) }
        @nodes[($pos - $FAILURE_PRETOKENS >= 0
          ? $pos - $FAILURE_PRETOKENS : 0) .. $pos - 1])
      . ">"
      . join('.', map { $self->getGrammaticalRole($_) }
        @nodes[$pos .. ($pos + $FAILURE_POSTTOKENS - 1 < $#nodes
          ? $pos + $FAILURE_POSTTOKENS - 1 : $#nodes)]);
    my $lexeme = node_location($nodes[$pos] || $nodes[$pos - 1] || $mathnode);
    my $indent = length($parsed) - 2; $indent = 8 if $indent > 8;
    Warn('not_parsed', $parsefail, $mathnode,
      "MathParser failed to match rule '$rule'",
      ($loc ? ($loc) : ()),
      ($parsed
        ? ($parsed, (' ' x $indent) . "> " . $toparse)
        : ("> " . $toparse)));
  }
  return; }

# used for debugging & failure reporting.
sub node_string {
  my ($node, $document) = @_;
  my $role = $node->getAttribute('role') || 'UNKNOWN';
  my $box = $document->getNodeBox($node);
  return ($box ? ToString($box) : text_form($node)) . "[[$role]]"; }

#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# Conversion to a less ambiguous, mostly-prefix form.
# Mostly for debugging information?
# Note that the nodes are true libXML nodes, already absorbed into the document
#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
sub text_form {
  my ($node) = @_;
  #  $self->textrec($node,0); }
  # Hmm, Something Weird is broken!!!!
  # With <, I get "unterminated entity reference" !?!?!?
  #  my $text= $self->textrec($node,0);
  my $text = textrec($node, undef);
  $text =~ s/</less/g;
  return $text; }

my %PREFIX_ALIAS = (    # [CONSTANT]
  SUPERSCRIPTOP => '^', SUBSCRIPTOP => '_', times          => => '*',
  'equals'      => '=', 'less-than' => '<', 'greater-than' => '>',
  'less-than-or-equals' => '<=', 'greater-than-or-equals' => '>=',
  'much-less-than'      => '<<', 'much-greater-than'      => '>>',
  'plus'                => '+',  'minus'                  => '-', 'divide' => '/');
# Put infix, along with `binding power'
my %IS_INFIX = (METARELOP => 1,    # [CONSTANT]
  RELOP         => 2,    ARROW       => 2,
  ADDOP         => 10,   MULOP       => 100, FRACOP => 100,
  SUPERSCRIPTOP => 1000, SUBSCRIPTOP => 1000);

sub textrec {
  my ($node, $outer_bp, $outer_name) = @_;
  return '[missing]' unless defined $node;
  $node = realizeXMNode($node);
  my $tag = getQName($node);
  $outer_bp   = 0  unless defined $outer_bp;
  $outer_name = '' unless defined $outer_name;
  if ($tag eq 'ltx:XMApp') {
    my $app_role = $node->getAttribute('role');
    my ($op, @args) = element_nodes($node);
    $op = realizeXMNode($op);
    if ($app_role && $app_role =~ /^FLOAT(SUB|SUPER)SCRIPT$/) {
      return ($1 eq 'SUPER' ? '^' : '_') . textrec($op); }
    else {
      my $name = ((getQName($op) eq 'ltx:XMTok') && getTokenMeaning($op)) || 'unknown';
      my ($bp, $string) = textrec_apply($name, $op, @args);
      return (($bp < $outer_bp) || (($bp == $outer_bp) && ($name ne $outer_name))
        ? '(' . $string . ')' : $string); } }
  elsif ($tag eq 'ltx:XMDual') {
    my ($content, $presentation) = element_nodes($node);
    return textrec($content, $outer_bp, $outer_name); }    # Just send out the semantic form.
  elsif ($tag eq 'ltx:XMTok') {
    my $name = getTokenMeaning($node);
    $name = 'Unknown' unless defined $name;
    return $PREFIX_ALIAS{$name} || $name; }
  elsif (($tag eq 'ltx:XMWrap') || ($tag eq 'ltx:XMCell')) {
    # ??
    return join('@', map { textrec($_) } element_nodes($node)); }
  elsif ($tag eq 'ltx:XMArray') {
    return textrec_array($node); }
  else {
    return '[' . (p_getValue($node) || '') . ']'; } }

sub textrec_apply {
  my ($name, $op, @args) = @_;
  my $role = $op->getAttribute('role') || 'Unknown';
  if (($role =~ /^(SUB|SUPER)SCRIPTOP$/) && (($op->getAttribute('scriptpos') || '') =~ /^pre\d+$/)) {
    # Note that this will likely get parenthesized due to high bp
    return (5000, textrec($op) . " " . textrec($args[1]) . " " . textrec($args[0])); }
  elsif (my $bp = $IS_INFIX{$role}) {
    # Format as infix.
    return ($bp, (scalar(@args) == 1    # unless a single arg; then prefix.
        ? textrec($op) . ' ' . textrec($args[0], $bp, $name)
        : join(' ' . textrec($op) . ' ', map { textrec($_, $bp, $name) } @args))); }
  elsif ($role eq 'POSTFIX') {
    return (10000, textrec($args[0], 10000, $name) . textrec($op)); }
  elsif ($name eq 'multirelation') {
    return (2, join(' ', map { textrec($_, 2, $name) } @args)); }
  else {
    return (500, textrec($op, 10000, $name) . '@(' . join(', ', map { textrec($_) } @args) . ')'); } }

sub textrec_array {
  my ($node) = @_;
  my $name = $node->getAttribute('meaning') || $node->getAttribute('name') || 'Array';
  my @rows = ();
  foreach my $row (element_nodes($node)) {
    push(@rows, '[' . join(', ', map { ($_->firstChild ? textrec($_->firstChild) : '') } element_nodes($row)) . ']'); }
  return $name . '[' . join(', ', @rows) . ']'; }

#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# Cute! Were it NOT for Sub/Superscripts, the whole parsing process only
# builds a new superstructure around the sequence of token nodes in the input.
# Thus, any internal structure is unchanged.
#  They get re-parented, but if the parse fails, we've only got to put them
# BACK into the original node, to recover the original arrangment!!!
# Thus, we don't have to clone, and deal with namespace duplication.
# ...
# EXCEPT, as I said, for sub/superscripts!!!!
#

#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# Constructors used in grammar
# All the tree construction in the grammar should come through these operations.
# We avoid mucking with the actual XML nodes (both to avoid modifying the original
# tree until we have a successful parse, and to avoid XML::LibXML cloning nightmares)
# We are converting XML nodes to array representation: [$tag, {%attr},@children]
# This means any inspection of nodes has to recognize that
#  * node may be in XML vs ARRAY representation
#  * node may be an XMRef to another node whose properties are the ones we should use.
#
# Also, when we are examining a node's properties (roles, fences, script positioning, etc)
# we should be careful to check for XMRef indirection and examine the properties
# of the node that was referred to.
# HOWEVER, we should construct our parse tree using (a clone of) the XMRef node,
# rather than (a clone of) the referred to node, so as to preserve identity.
#%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
# We're currently keeping the id's on the nodes as they get cloned,
# since they'll (maybe) replace the unparsed nodes.
# However, if we consider multiple parses or preserving both parsed & unparsed,
# we may have to do some adaptation and id shifting.
# ================================================================================

# ================================================================================
# Low-level accessors
sub Lookup {
  my ($lexeme) = @_;
  return $$LaTeXML::MathParser::LEXEMES{$lexeme}; }

# The following accessors work on both the LibXML and ARRAY representations
# but they do NOT automatically dereference XMRef!
sub p_getValue {
  my ($node) = @_;
  if (!defined $node) {
    return; }
  elsif (ref $node eq 'XML::LibXML::Element') {
    my $x;
    return (($x = $node->textContent) ne '' ? $x    # get content, or fall back to name
      : (defined($x = $node->getAttribute('name')) ? $x
        : undef)); }
  elsif (ref $node eq 'ARRAY') {
    my ($op, $attr, @args) = @$node;
    if (@args) {
      return join('', grep { defined $_ } map { p_getValue($_) } @args); }
    else {
      return $$node[1]{name}; } }
  elsif (ref $node eq 'XML::LibXML::Text') {
    return $node->textContent; }
  else {                                            # ????
    return $node; } }

sub p_getTokenMeaning {
  my ($item) = @_;
  if (!defined $item) {
    return; }
  elsif (ref $item eq 'ARRAY') {
    my ($op, $attr, @args) = @$item;
    return $$attr{meaning} || $$attr{name} || $args[0] || $$attr{role}; }
  elsif (ref $item eq 'XML::LibXML::Element') {
    return getTokenMeaning($item); } }

sub p_getAttribute {
  my ($item, $key) = @_;
  if (!defined $item) {
    return; }
  elsif (ref $item eq 'ARRAY') {
    return $$item[1]{$key}; }
  elsif (ref $item eq 'XML::LibXML::Element') {
    return $item->getAttribute($key); } }

sub p_element_nodes {
  my ($item) = @_;
  if (!defined $item) {
    return (); }
  elsif (ref $item eq 'ARRAY') {
    my ($op, $attr, @args) = @$item;
    return grep { ref $_ } @args; }
  elsif (ref $item eq 'XML::LibXML::Element') {
    return element_nodes($item); } }

sub p_getQName {
  my ($item) = @_;
  if (!defined $item) {
    return; }
  elsif (ref $item eq 'ARRAY') {
    return $$item[0]; }
  elsif (ref $item eq 'XML::LibXML::Element') {
    return getQName($item); } }

# Make a new Token node with given name, content, and attributes.
# $content is an array of nodes (which may need to be cloned if still attached)
sub New {
  my ($meaning, $content, %attributes) = @_;
  my %attr = ();
  $attr{meaning} = $meaning if $meaning;
  foreach my $key (sort keys %attributes) {
    my $value = p_getValue($attributes{$key});
    $attr{$key} = $value if defined $value; }
  if (!$attr{font}) {
    $attr{font} = ($content && $content =~ /\S/
      ? LaTeXML::Common::Font->textDefault->specialize($content)
      : LaTeXML::Common::Font->new()); }
  return ['ltx:XMTok', {%attr}, ($content ? ($content) : ())]; }

# Like New(), but concatenating several symbols into one new one.
# This accounts for the symbols "disappearing", being replaced by one,
# and thus any references to the old one have to be adjusted ... or removed
# I'm a little concerned that we should be defering this till a successful parse?
sub CatSymbols {
  my ($symbol1, $symbol2, $meaning, $content, %attributes) = @_;
  my $new = New($meaning, $content, %attributes);
  my $repl;
  my $symbols = [$symbol1, $symbol2];
  my $doc = $LaTeXML::MathParser::DOCUMENT;
  foreach my $symbol (@$symbols) {
    if (my $id = p_getAttribute(realizeXMNode($symbol), 'xml:id')) {
      if (!$repl) {
        my $newid = $doc->modifyID($id);
        $$new[1]{'xml:id'} = $newid;
        ReplacedBy($symbol, $new);
        $repl = 1; }
      else {
        foreach my $r ($doc->findnodes("descendant-or-self::ltx:XMRef[\@idref='$id']")) {
          $doc->removeNode($r); } }    # ? Hopefully this is safe.
    } }
  return $new; }

# Some handy shorthands.
sub Absent {
  return New('absent'); }

sub InvisibleTimes {
  return New('times', "\x{2062}", role => 'MULOP', font => LaTeXML::Common::Font->new()); }

sub InvisibleComma {
  return New(undef, "\x{2063}", role => 'PUNCT', font => LaTeXML::Common::Font->new()); }

# Get n-th arg of an XMApp.
# However, this is really only used to get the script out of a sub/super script
sub Arg {
  my ($node, $n) = @_;
  my $onode = $node;
  $node = realizeXMNode($node);
  if (ref $node eq 'ARRAY') {
    return $$node[$n + 2]; }
  else {
    my @args = element_nodes($node);
    my $nth  = $args[$n];
    # Tricky case: if $node is an XMRef, we'll want to reference the SUB node too
    # and not just use it directly; else that node will be duplicated in both branches of XMDual
    if ($nth && !$node->isSameNode($onode)) {
      return LaTeXML::Package::createXMRefs($LaTeXML::MathParser::DOCUMENT, $nth); }
    else {
      return $args[$n]; } } }    # will get cloned if/when needed.

# Add more attributes to a node.
# Values can be strings or nodes whose text content is used.
# Note that it avoids changing the underlying XML, so it may return a new object!
sub Annotate {
  my ($node, %attributes) = @_;
  my %attrib = ();
  # first scan & convert any attributes, to make sure there really are any values.
  foreach my $attr (keys %attributes) {
    my $value = p_getValue($attributes{$attr});
    $attrib{$attr} = $value if defined $value; }
  # Hmm.... maybe we need to merge some things, open close?
  if (keys %attrib) {    # Any attributes to assign?
                         # If we've gotten a real XML node, convert to array representation
                         # We do NOT want to modify any of the original XML!!!!
    if (ref $node ne 'ARRAY') {
      $node = [getQName($node),
        { map { (getQName($_) => $_->getValue) }
            grep { $_->nodeType == XML_ATTRIBUTE_NODE } $node->attributes },
        $node->childNodes]; }
    my $qname = $$node[0];
    # Remove any attributes that aren't allowed!!!
    foreach my $k (keys %attrib) {
      delete $attrib{$k} unless $k =~ /^_/ || $LaTeXML::MathParser::DOCUMENT->canHaveAttribute($qname, $k); }
    # Make sure font is "Appropriate", if we're creating a new token
    if ($attrib{_font} && ($qname eq 'ltx:XMTok')) {
      my $content = join('', @$node[2 .. $#$node]);
      if ((!defined $content) || ($content eq '')) {
        delete $attrib{_font}; }    # No font needed
      elsif (my $font = $LaTeXML::MathParser::DOCUMENT->decodeFont($attrib{_font})) {
        delete $attrib{_font};
        $attrib{font} = $font->specialize($content); } }
    map { $$node[1]{$_} = $attrib{$_} } keys %attrib; }    # Now add them.
  return $node; }

# ================================================================================
# Mid-level constructors

# Apply $op to the list of arguments
# args may be array-rep or lexemes (or nodes?)
sub Apply {
  my ($op, @args) = @_;
  my $font = p_getAttribute($op, '_font');
  return ['ltx:XMApp', { ($font ? (_font => $font) : ()) }, $op, @args]; }

# Apply $op to a `delimited' list of arguments of the form
#     open, expr (punct expr)* close
# after extracting the opening and closing delimiters, and the separating punctuation
# Generate an XMDual, so that any styling of delimiters & punctuation is preserved.
sub ApplyDelimited {
  my ($op, @stuff) = @_;
  my $open  = $stuff[0];
  my $close = $stuff[-1];
  my ($seps, @args) = extract_separators(@stuff[1 .. $#stuff - 1]);
  return ['ltx:XMDual', {},
    Apply(LaTeXML::Package::createXMRefs($LaTeXML::MathParser::DOCUMENT, $op),
      LaTeXML::Package::createXMRefs($LaTeXML::MathParser::DOCUMENT, @args)),
    Apply($op, ['ltx:XMWrap', {}, @stuff])]; }

# This is similar, but "interprets" a delimited list as being the
# application of some operator to the items in the list.
sub InterpretDelimited {
  my ($op, @stuff) = @_;
  my $open  = $stuff[0];
  my $close = $stuff[-1];
  my ($seps, @args) = extract_separators(@stuff[1 .. $#stuff - 1]);
  return ['ltx:XMDual', {},
    Apply($op, LaTeXML::Package::createXMRefs($LaTeXML::MathParser::DOCUMENT, @args)),
    ['ltx:XMWrap', {}, @stuff]]; }

# Given a sequence of operators, form the nested application op(op(...(arg)))
sub recApply {
  my (@ops) = @_;
  return (scalar(@ops) > 1 ? Apply(shift(@ops), recApply(@ops)) : $ops[0]); }

# Given  alternating expressions & separators (punctuation,...)
# extract the separators as a concatenated string,
# returning (separators, args...)
sub extract_separators {
  my (@stuff) = @_;
  my ($punct, @args);
  if (@stuff) {
    push(@args, shift(@stuff));    # Grab 1st expression
    while (@stuff) {               # Expecting pairs of punct, expression
      my $p = realizeXMNode(shift(@stuff));
      $punct .=
        ($punct ? ' ' : '')        # Delimited by SINGLE SPACE!
        . spacingToString(getXMHintSpacing(p_getAttribute($p, 'lpadding')))
        . p_getValue($p)
        . spacingToString(getXMHintSpacing(p_getAttribute($p, 'rpadding')));
      push(@args, shift(@stuff)); } }    # Collect the next expression.
  return ($punct, @args); }

# ================================================================================
# Some special cases

# This specifies the "meaning" of things within a pair
# of open/close delimiters, depending on the number of things.
# Really should be customizable?
# Note that these are all Best Guesses, but really can have
# alternate interpretations depending on context, field, etc.
# Question: Is there enough context to guess better?
# For example, whether (a,b) is an interval or list?
#  (both could reasonably be preceded by \in )
my %balanced = (    # [CONSTANT]
  '(' => ')', '[' => ']', '{' => '}',
  '|' => '|', '||' => '||',
  "\x{230A}" => "\x{230B}",    # lfloor, rfloor
  "\x{2308}" => "\x{2309}",    # lceil, rceil
  "\x{2329}" => "\x{232A}",    # angle brackets; NOT mathematical, but balance in case they show up.
  "\x{27E8}" => "\x{27E9}",    # angle brackets (preferred)
  "\x{2225}" => "\x{2225}",    # lVert, rVert
);
# For enclosing a single object
# Note that the default here is just to put open/closed attributes on the single object
my %enclose1 = (    # [CONSTANT]
  '{@}'   => 'set',                                   # alternatively, just variant parentheses
  '|@|'   => 'absolute-value',
  '||@||' => 'norm', "\x{2225}@\x{2225}" => 'norm',
  "\x{230A}@\x{230B}" => 'floor',
  "\x{2308}@\x{2309}" => 'ceiling',
  '<@>'               => 'expectation',               # or just average?
  '<@|'               => 'bra', '|@>' => 'ket');
# For enclosing more than 2 objects; the punctuation is significant too
my %enclose2 = (                                      # [CONSTANT]
  '(@,@)' => 'open-interval',                                           # alternatively, just a list
  '[@,@]' => 'closed-interval',
  '(@,@]' => 'open-closed-interval', '[@,@)' => 'closed-open-interval',
  '{@,@}' => 'set',                                                     # alternatively, just a list ?
);
# For enclosing more than 2 objects.
# assume 1st punct? or should we check all are same?
my %encloseN = (    # [CONSTANT]
  '(@,@)' => 'vector', '{@,@}' => 'set',);

sub isMatchingClose {
  my ($open, $close) = @_;
  my $oname  = p_getValue(realizeXMNode($open));
  my $cname  = p_getValue(realizeXMNode($close));
  my $expect = $oname && $balanced{$oname};
  return (defined $expect) && (defined $cname) && ($expect eq $cname); }

# Given a delimited sequence: open expr (punct expr)* close
# (OR, an empty sequence open close)
# Convert it into the appropriate thing, depending on the specific open & close used.
# Generate an XMDual to preserve any styling of delimiters and punctuation.
sub Fence {
  my (@stuff) = @_;
  # Peak at delimiters to guess what kind of construct this is.
  my $nargs = scalar(@stuff);
  Error("expected", "arguments", undef,
    "Even number of arguments to Fence(); should be of form open,expr,(punct,expr)*,close",
    "got " . join(' ', map { ToString($_) } @stuff))
    if ($nargs != 2) && (($nargs % 2) == 0);    # either empty or odd number
  my ($open, $close) = (realizeXMNode($stuff[0]), realizeXMNode($stuff[-1]));
  my $o  = p_getValue($open);
  my $c  = p_getValue($close);
  my $n  = int(($nargs - 2 + 1) / 2);
  my @p  = map { p_getValue(realizeXMNode(@stuff[2 * $_])) } 1 .. $n - 1;
  my $op = ($n == 0
    ? 'list'                                    # ?
    : ($n == 1
      ? $enclose1{ $o . '@' . $c }
      : ($n == 2
        ? ($enclose2{ $o . '@' . $p[0] . '@' . $c } || 'list')
        : ($encloseN{ $o . '@' . $p[0] . '@' . $c } || 'list'))));
  $op = 'delimited-' . $o . $c unless defined $op;
  if (($n == 1) && ($op eq 'delimited-()')) {    # Hopefully, can just ignore the parens?
    return ['ltx:XMDual', {},
      LaTeXML::Package::createXMRefs($LaTeXML::MathParser::DOCUMENT, $stuff[1]),
      ['ltx:XMWrap', {}, @stuff]]; }
  else {
    return InterpretDelimited(New($op), @stuff); } }

# NOTE: It might be best to separate the multiple Formulae into separate XMath's???
# but only at the top level!
sub NewFormulae {
  my (@stuff) = @_;
  if (scalar(@stuff) == 1) {
    return $stuff[0]; }
  else {
    my ($seps, @formula) = extract_separators(@stuff);
    return ['ltx:XMDual', {},
      Apply(New('formulae'),
        LaTeXML::Package::createXMRefs($LaTeXML::MathParser::DOCUMENT, @formula)),
      ['ltx:XMWrap', {}, @stuff]]; } }

# A Formula is an alternation of expr (relationalop expr)*
# It presumably would be equivalent to (expr1 relop1 expr2) AND (expr2 relop2 expr3) ...
# But, I haven't figured out the ideal prefix form that can easily be converted to presentation.
sub NewFormula {
  my (@args) = @_;
  my $n = scalar(@args);
  if ($n == 1) {
    return $args[0]; }
  elsif ($n == 3) {
    return Apply($args[1], $args[0], $args[2]); }
  else {
    return Apply(New('multirelation'), @args); } }

sub NewList {
  my (@stuff) = @_;
  if (@stuff == 1) {
    return $stuff[0]; }
  else {
    my ($seps, @items) = extract_separators(@stuff);
    return ['ltx:XMDual', {},
      Apply(New('list'),
        LaTeXML::Package::createXMRefs($LaTeXML::MathParser::DOCUMENT, @items)),
      ['ltx:XMWrap', {}, @stuff]]; } }

# Given alternation of expr (addop expr)*, compose the tree (left recursive),
# flattenning portions that have the same operator
# ie. a + b + c - d  =>  (- (+ a b c) d)
sub LeftRec {
  my ($arg1, @more) = @_;
  if (@more) {
    my $op     = shift(@more);
    my $opname = p_getTokenMeaning(realizeXMNode($op));
    my @args   = ($arg1, shift(@more));
    while (@more && isSameExpr($op, $more[0])) {
      ReplacedBy($more[0], $op, 1);
      shift(@more);
      push(@args, shift(@more)); }
    return LeftRec(Apply($op, @args), @more); }
  else {
    return $arg1; } }

# Like apply($op,$arg1,$arg2), but if $op is 'same' as the operator in $arg1,
# then combine as an nary apply of $op to $arg1's arguments and $arg2.
sub ApplyNary {
  my ($op, $arg1, $arg2) = @_;
  my $rop       = realizeXMNode($op);
  my $opname    = p_getTokenMeaning($rop) || '__undef_meaning__';
  my $opcontent = p_getValue($rop) || '__undef_content__';
  my @args      = ();
  if (p_getQName($arg1) eq 'ltx:XMApp') {
    my ($op1, @args1) = p_element_nodes($arg1);
    my $rop1 = realizeXMNode($op1);
    if (isSameExpr($rop, $rop1)
      # Check that arg1 isn't wrapped, fenced or enclosed in some restrictive way
      # Especially an ID! (but really only important if the id is referenced somewhere?)
      && !(grep { p_getAttribute(realizeXMNode($arg1), $_) } qw(enclose xml:id))) {
      # Note that $op1 GOES AWAY!!!
      ReplacedBy($op1, $rop, 1);
      push(@args, @args1); }
    else {
      push(@args, $arg1); } }
  else {
    push(@args, $arg1); }
  return Apply($op, @args, $arg2); }

# Usually we just expect to compare a token + to another.
# but want (to some extent) to deal with embellished operators (eg. sub, sup...)
# Rather involved if we want to try to do it "Right".
sub isSameExpr {
  my ($op1, $op2) = @_;
  $op1 = realizeXMNode($op1);
  $op2 = realizeXMNode($op2);
  my $tag1 = getQName($op1);
  my $tag2 = getQName($op2);
  # Either both are tokens,
  # OR both have structure, but then we need to compare the children!!!!
  # First check, same top-level and critical attributes
  return unless
    ($tag1 eq $tag2)
    && ((p_getTokenMeaning($op1) || '__undef_meaning__')
    eq (p_getTokenMeaning($op2) || '__undef_meaning__'))
    && ((p_getValue($op1) || '__undef_content__')
    eq (p_getValue($op2) || '__undef_content__'))
    # Check that ops are used in same way.
    && ((p_getAttribute($op1, 'mathstyle') || '<none>')
    eq (p_getAttribute($op2, 'mathstyle') || '<none>'));
  if ($tag1 eq 'ltx:XMTok') { return 1; }    # If tokens, they match
  else {
    my @ch1 = p_element_nodes($op1);
    my @ch2 = p_element_nodes($op2);
    my $n   = scalar(@ch1);
    return unless $n == scalar(@ch2);
    foreach my $i (0 .. $n - 1) {
      return unless isSameExpr($ch1[$i], $ch1[$i]); }
    return 1; } }

# There are several cases where parsing a formula will rearrange nodes
# such that some nodes will no-longer be used.  For example, when
# converting a nested set of infix + into a single n-ary sum.
# In effect, all those excess +'s are subsumed by the single first one.
# It may be, however, that those lost nodes are referenced (XMRef) from the
# other branch of an XMDual, and those references should be updated to refer
# to the single node replacing the lost ones.
# This function records that replacement, and the top-level parser fixes up the tree.
# NOTE: There may be cases (in the Grammar, eg) where punctuation & ApplyOp's
# get lost completely? Watch out for this!
# If $isdup, assume the two trees are equivalent structure,
# and so mark corresponding internal nodes as "replaced" as well.
sub ReplacedBy {
  my ($lostnode, $keepnode, $isdup) = @_;
  return unless ref $lostnode && ref $keepnode;
  if (my $lostid = p_getAttribute($lostnode, 'xml:id')) {
    # Could be we want to generate an id for $keepnode, here?
    if (my $keepid = p_getAttribute($keepnode, 'xml:id')) {
      #      print STDERR "LOST $lostid use instead $keepid\n";
      $$LaTeXML::MathParser::LOSTNODES{$lostid} = $keepid; }
    else {
      print STDERR "LOST $lostid but no replacement!\n"; } }

  # The following recurses into the two trees,
  # This is on the assumption that they are "equivalent" trees
  if ($isdup) {
    my @lostkids = p_element_nodes($lostnode);
    my @keepkids = p_element_nodes($keepnode);
    my $n        = scalar(@lostkids);
    my $m        = scalar(@keepkids);
    if ($n != $m) {
      Error("unexpected", "nodes", undef, "Nodes aren't the same structure ($n vs $m)",
        "Old: " . printNode($lostnode),
        "New: " . printNode($keepnode)
      );
      $n = $m if $m < $n; }
    foreach my $i (0 .. $n - 1) {
      ReplacedBy($lostkids[$i], $keepkids[$i], $isdup); } }
  return; }

# ================================================================================
# Construct an appropriate application of sub/superscripts
# This accounts for script positioning:
#   Whether it precedes (float), is over/under (if base requests),
# or follows (normal case), along with whether sub/super.
#   the alignment of multiple sub/superscripts derived from the binding level when created.
# scriptpos = (pre|mod|post) number; where number is the binding-level.
# If $pos is given (pre|mid|post), it overrides the position implied by the script
sub NewScript {
  my ($base, $script, $pos) = @_;
  my $role;
  my $rbase   = realizeXMNode($base);
  my $rscript = realizeXMNode($script);
  my $ibase   = $rbase;
  # Get "inner" (content) base, if the base is a dual
  if (p_getQName($rbase) eq 'ltx:XMDual') {
    ($ibase) = p_element_nodes($rbase); }
  my ($bx, $bl) = (p_getAttribute($ibase,   'scriptpos') || 'post') =~ /^(pre|mid|post)?(\d+)?$/;
  my ($sx, $sl) = (p_getAttribute($rscript, 'scriptpos') || 'post') =~ /^(pre|mid|post)?(\d+)?$/;
  my ($mode, $y) = p_getAttribute($rscript, 'role') =~ /^(FLOAT|POST)?(SUB|SUPER)SCRIPT$/;
  my $x = ($pos ? $pos : ($mode eq 'FLOAT' ? 'pre' : $bx || 'post'));
  my $lpad = ($x eq 'pre') && p_getAttribute($rscript, 'lpadding');
  my $rpad = ($x ne 'pre') && p_getAttribute($rscript, 'rpadding');
  my $t;
  my $l = $sl || $bl ||
    (($t = $LaTeXML::MathParser::DOCUMENT->getNodeBox($script))
    && ($t->getProperty('level'))) || 0;
  # If the INNER script was a floating script (ie. {}^{x})
  # we'll NOT want this one to stack over it so bump the level.
  my $bumped;
  if (p_getAttribute($rbase, '_wasfloat')) {
    $l++; $bumped = 1 }
  elsif (my $innerl = p_getAttribute($rbase, '_bumplevel')) {
    $l = $innerl; }
  my $app = Apply(New(undef, undef, role => $y . 'SCRIPTOP', scriptpos => "$x$l"),
    $base, Arg($script, 0));
  # Record whether this script was a floating one
  $$app[1]{_wasfloat}  = 1  if $mode eq 'FLOAT';
  $$app[1]{_bumplevel} = $l if $bumped;
  $$app[1]{scriptpos} = $bx   if $bx   && ($bx ne 'post');
  $$app[1]{lpadding}  = $lpad if $lpad && !$$app[1]{lpadding};    # better to add?
  $$app[1]{rpadding}  = $rpad if $rpad && !$$app[1]{rpadding};    # better to add?
  return $app; }

# Basically, like NewScript, but decorates an operator with sub/superscripts
# (with vague unknown implications for meaning?)
# but which will preserve the role (& meaning?)
sub DecorateOperator {
  my ($op, $script) = @_;
  my $decop   = NewScript($op, $script);
  my $rop     = realizeXMNode($op);
  my $role    = p_getAttribute($rop, 'role');
  my $meaning = p_getAttribute($rop, 'meaning');
  return Annotate($decop, role => $role, meaning => $meaning); }

sub NewEvalAt {
  my ($base, $vertbar, $lower, $upper) = @_;
  my $pres = ['ltx:XMWrap', {}, $base, Annotate($vertbar, role => 'CLOSE')];
  $pres = NewScript($pres, $lower) if $lower;
  $pres = NewScript($pres, $upper) if $upper;
  return ['ltx:XMDual', {},
    ['ltx:XMApp', {},
      ['ltx:XMTok', { meaning => 'evaluated-at' }],
      LaTeXML::Package::createXMRefs($LaTeXML::MathParser::DOCUMENT,
        $base,
        ($lower ? (Arg($lower, 0)) : ()),
        ($upper ? (Arg($upper, 0)) : ()))],
    $pres]; }

# ================================================================================
# A "notation" is a language construct or set thereof.

# Called from the grammar to record the fact that a notation was seen.
sub SawNotation {
  my ($notation, $node) = @_;
  $LaTeXML::MathParser::SEEN_NOTATIONS{$notation} = 1;
  return 1; }

# Called by the grammar to determine whether we should try productions
# which involve the given notation.
sub IsNotationAllowed {
  my ($notation) = @_;
  return ($LaTeXML::MathParser::DISALLOWED_NOTATIONS{$notation} ? undef : 1); }

# ================================================================================

# Note that an UNKNOWN token may have been used as a function.
# For simplicity in the grammar, we accept a token that has sub|super scripts applied.
sub MaybeFunction {
  my ($token) = @_;
  $token = realizeXMNode($token);
  my $self = $LaTeXML::MathParser::PARSER;
  while (p_getQName($token) eq 'ltx:XMApp') {
    $token = Arg($token, 1); }
  my $name = token_prettyname($token);
  # DANGER!!
  # We want to be using Annotate here, but we're screwed up by the
  # potential "embellishing" of the function token.
  # (ie. the descent above past all XMApp's)
  $token->setAttribute('possibleFunction', 'yes');
  $$self{maybe_functions}{$name}++
    if $LaTeXML::MathParser::STRICT && !$$self{suspicious_tokens}{$token};
  $$self{suspicious_tokens}{$token} = 1;
  return; }

# ================================================================================
1;

__END__

=pod 

=head1 NAME

C<LaTeXML::MathParser> - parses mathematics content

=head1 DESCRIPTION

C<LaTeXML::MathParser> parses the mathematical content of a document.
It uses L<Parse::RecDescent> and a grammar C<MathGrammar>.

=head2 Math Representation

Needs description.

=head2 Possibile Customizations

Needs description.

=head2 Convenience functions

The following functions are exported for convenience in writing the
grammar productions.

=over 4

=item C<< $node = New($name,$content,%attributes); >>

Creates a new C<XMTok> node with given C<$name> (a string or undef),
and C<$content> (a string or undef) (but at least one of name or content should be provided),
and attributes.

=item C<< $node = Arg($node,$n); >>

Returns the C<$n>-th argument of an C<XMApp> node;
0 is the operator node.

=item C<< Annotate($node,%attributes); >>

Add attributes to C<$node>.

=item C<< $node = Apply($op,@args); >>

Create a new C<XMApp> node representing the application of the node
C<$op> to the nodes C<@args>.

=item C<< $node = ApplyDelimited($op,@stuff); >>

Create a new C<XMApp> node representing the application of the node
C<$op> to the arguments found in C<@stuff>.  C<@stuff> are 
delimited arguments in the sense that the leading and trailing nodes
should represent open and close delimiters and the arguments are
separated by punctuation nodes.

=item C<< $node = InterpretDelimited($op,@stuff); >>

Similar to C<ApplyDelimited>, this interprets sequence of
delimited, punctuated items as being the application of C<$op> to those items.

=item C<< $node = recApply(@ops,$arg); >>

Given a sequence of operators and an argument, forms the nested
application C<op(op(...(arg)))>>.

=item C<< $node = InvisibleTimes; >>

Creates an invisible times operator.

=item C<< $boole = isMatchingClose($open,$close); >>

Checks whether C<$open> and C<$close> form a `normal' pair of
delimiters, or if either is ".".

=item C<< $node = Fence(@stuff); >>

Given a delimited sequence of nodes, starting and ending with open/close delimiters,
and with intermediate nodes separated by punctuation or such, attempt to guess what
type of thing is represented such as a set, absolute value, interval, and so on.

This would be a good candidate for customization!

=item C<< $node = NewFormulae(@stuff); >>

Given a set of formulas, construct a C<Formulae> application, if there are more than one,
else just return the first.

=item C<< $node = NewList(@stuff); >>

Given a set of expressions, construct a C<list> application, if there are more than one,
else just return the first.

=item C<< $node = LeftRec($arg1,@more); >>

Given an expr followed by repeated (op expr), compose the left recursive tree.
For example C<a + b + c - d> would give C<(- (+ a b c) d)>>

=item C<< MaybeFunction($token); >>

Note the possible use of C<$token> as a function, which may cause incorrect parsing.
This is used to generate warning messages.

=back

=head1 AUTHOR

Bruce Miller <bruce.miller@nist.gov>

=head1 COPYRIGHT

Public domain software, produced as part of work done by the
United States Government & not subject to copyright in the US.

=cut

