struct prb_node *y = w->prb_link[0];
y->prb_color = PRB_BLACK;
w->prb_color = PRB_RED;
w->prb_link[0] = y->prb_link[1];
y->prb_link[1] = w;
if (w->prb_link[0] != NULL)
  w->prb_link[0]->prb_parent = w;
w = f->prb_link[1] = y;
w->prb_link[1]->prb_parent = w;
