w->prb_color = f->prb_color;
f->prb_color = PRB_BLACK;
w->prb_link[0]->prb_color = PRB_BLACK;

f->prb_link[0] = w->prb_link[1];
w->prb_link[1] = f;
g->prb_link[g->prb_link[0] != f] = w;

w->prb_parent = f->prb_parent;
f->prb_parent = w;
if (f->prb_link[0] != NULL)
  f->prb_link[0]->prb_parent = f;
