sclass DoubleKeyedMap<A, B, C> {

  new Map<A, Map<B, C>> map1;

  new Map<B, Map<A, C>> map2;

  Map<B, C> getA(A a) { ret map1.get(a); }

  Map<A, C> getB(B b) { ret map2.get(b); }

  Cl<C> valuesForA(A a) { ret values(getA(a)); }

  Cl<C> valuesForB(B b) { ret values(getB(b)); }

  Set<B> bsForA(A a) { ret keys(getA(a)); }

  Set<A> asForB(B b) { ret keys(getB(b)); }

  C get(A a, B b) { ret mapGet(map1.get(a), b); }

  C get(Pair<A, B> p) { ret p == null ? null : get(p.a, p.b); }

  C put(A a, B b, C c) {

    mapGetOrCreateHashMap(map1, a).put(b, c);

    ret mapGetOrCreateHashMap(map2, b).put(a, c);

  }

  C put(Pair<A, B> p, C c) {

    ret put(p.a, p.b, c);

  }

  C remove(A a, B b) {

    nestedMapRemove(map1, a, b);

    ret nestedMapRemove(map2, b, a);

  }

  Set<A> aKeys() { ret keys(map1); }

  Set<B> bKeys() { ret keys(map2); }

  void removeAllA(Iterable<A> l) { fOr (A a : l) removeA(a); }

  void removeA(A a) {

    fOr (B b : cloneList(bsForA(a))) remove(a, b);

  }

}