code: private Map<Transition<A, S>, N> predecessorsMap(S current){
          //Map<Transition, Node> containing predecessors relations
          Map<Transition<A, S>, N> mapPredecessors = new HashMap<Transition<A, S>, N>();
          //Fill with non-null pairs of <Transition, Node>
          for (Transition<A, S> predecessor : predecessorFunction.transitionsFrom(current)) {
              N predecessorNode = visited.get(predecessor.getState());
              if (predecessorNode != null) {
                  mapPredecessors.put(predecessor, predecessorNode);
              }
          }
          return mapPredecessors;
      }
docs: Retrieves a map with the predecessors states and the node associated
  to each predecessor state.
  
  @param current current state to calculate predecessors of
  @return map pairs of <state, node> with the visited predecessors of the state