register_precedence (graphid first, graphid snd)
{
  if (!prec_nodes)
    init_prec_nodes ();
  register_precedence_second_symbol (&(prec_nodes[first]->succ), snd);
  register_precedence_second_symbol (&(prec_nodes[snd]->pred), first);
}
register_precedence (graphid first, graphid snd)
{
  if (!prec_nodes)
    init_prec_nodes ();
  register_precedence_second_symbol (&(prec_nodes[first]->succ), snd);
  register_precedence_second_symbol (&(prec_nodes[snd]->pred), first);
}