public Ab<E> fillDret() throws ArbreException {
   if (this.arrel == null) throw new ArbreException("Ab buit no te fills");
   AbEnll<E> retorn = new AbEnll<E>();
   retorn.arrel = this.arrel.dret;
   return retorn;
 }
 public Ab<E> fillEsquerre() throws ArbreException {
   if (this.arrel == null) throw new ArbreException("Ab buit no te fills");
   AbEnll<E> resultat = new AbEnll<E>();
   resultat.arrel = this.arrel.esq;
   return resultat;
 }