code: @Override
      public int compareTo(ADStarNodeImpl<A, S, C> o) {
          return this.key.compareTo(o.key);
      }
docs: Compares ADSTarNode instances attending to their {@link es.usc.citius.hipster.model.ADStarNode.Key}
  values.
  
  @param o ADStarNode instance
  @return usual comparison value