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