sclass AllOnAll_probabilistic_forTransformers implements Producer<Pair<F1, O>> {

  replace A with F1.

  replace B with O.

  // We're grouping both a's and b's by the type accepted by the transformers.

  // Of course things are never this easy because types have subtypes...

  new MultiMap<Class, A> aList;

  new MultiMap<Class, B> bList;

  LPair<Cl<A>, Cl<B>> todo = new LinkedList; // this is still non-probabilistic

  MultiSetMap<Double, Pair<A, B>> todo2 = multiSetMap_outerDescTreeMap_innerLinkedHashSet();

  persistently swappable double probabilityForPair(A a, B b) { ret 1.0; }

  void addA(A a) { newA(a); }

  void addAIfNotNull(A a) { if (a != null) newA(a); }

  synchronized void newA(A a) {

    add(aList, a);

    multiSetCombine(bList.get(classesForA(a));

    IntRange rB = intRange(0, l(bList));

    if (empty(rB)) ret;

    addPair(todo, intRange_last(aList), rB);

  }

  synchronized void newAs(Iterable<A> l) {

    fOr (A a : l) newA(a);

  }

  void addB(B b) { newB(b); }

  void addBIfNotNull(B b) { if (b != null) newB(b); }

  synchronized void newB(B b) {

    add(bList, b);

    IntRange rA = intRange(0, l(aList));

    if (empty(rA)) ret;

    addPair(todo, rA, intRange_last(bList));

  }

  synchronized void newBs(Iterable<B> l) {

    fOr (B b : l) newB(b);

  }

  public synchronized Pair<A, B> next() {

    Pair<IntRange> p;

    while ((p = popFirst(todo)) != null)

      for (A a : subList(aList, p.a))

        for (B b : subList(bList, p.b))

          addAPair(a, b, probabilityForPair(a, b));

    ret popFirstValueFromMultiSetMap(todo2);

  }

  private void addAPair(A a, B b, double probability) {

    if (probability > 0)

      todo2.put(probability, pair(a, b));

  }

  synchronized L cloneBList() {

    ret clonedList(bList);

  }

  L<A> getAs() { ret aList; }

  L<B> getBs() { ret bList; }

}