code: @SuppressWarnings("unchecked") //suppress warnings to return an ADStarNode instead of Node, which is the inherited return type from parent
      @Override
      public ADStarNodeImpl<A, S, C> previousNode() {
          return (ADStarNodeImpl<A, S, C>) previousNode;
      }
docs: Method to retrieve the parent ADStarNode, the same
  retrieved by {@link es.usc.citius.hipster.model.Transition#getFromState()}.
  
  @return parent {@link ADStarNodeImpl},