sclass BreadthFirstPathFinder<A> implements Steppable {
  new LinkedList<A> queue;
  MultiMap<A> links;
  IF1<A, Cl<A>> getChildren; // must set
  
  *() {}
  *(IF1<A, Cl<A>> *getChildren) {}
  *(IF1<A, Cl<A>> *getChildren, A startNode) { add(startNode); }
  
  void add(A a, A prev default null) {
    if (!links.containsKey(a)) {
      links.put(a, prev);
      queue.add(a);
    }
  }
  
  public bool step() {
    ping();
    if (empty(queue)) false;
    A a = popFirst(queue);
    fOr (A b : getChildren.get(a))
      add(b, a);
    true;
  }
  
  bool nodeReached(A a) {
    ret links.containsKey(a);
  }
  
  Cl<A> getBackLinks(A a) {
    ret links.get(a);
  }
}