code: public static HeuristicFunction<City, Double> heuristicFunction(){
          return new HeuristicFunction<City, Double>() {
              @Override
              public Double estimate(City state) {
                  return heuristics().get(state);
              }
          };
      }
docs: Heuristic function required to define search problems to be used with Hipster.
  @see es.usc.citius.hipster.model.problem.SearchProblem
  @return {@link es.usc.citius.hipster.model.function.HeuristicFunction} with the {@link #heuristics()} values.