// B is the type of object we handle
sclass TransformersOnObjects<B> implements Steppable {
  // using the oldschool type F1 here because it's a bit easier to handle
  new AllOnAll<F1, B> allOnAll;
  new Set<B> bs;
  
  AllOnAllOperation<F1, B> op = new(allOnAll,
    (a, b) -> {
      if (!callableOn_nonSynthetic(a, "get", b)) ret;
      O out = callFOpt(a, b);
      if (out cast Iterable)
        for (O x : out) addObject((B) x);
      else
        addObject((B) out);
    });

  void add(B b) { addObject(b); }
  
  void addObject(B b) {
    if (addIfNotNull(bs, b))
      allOnAll.addB(b);
  }
  
  void addTransformer(F1 f) {
    allOnAll.addA(f);
  }
  
  void addTransformer(F2 f) {
    addTransformer(func(O a) {
      addTransformer(func(O b) {
        callableOn_nonSynthetic(f, "get", a, b) ? f.get(a, b) : null
      });
      null;
    });
  }
  
  public bool step() { ret op.step(); }
  
  L<B> getObjects() { ret allOnAll.getBs(); }
}