static <B, A> MultiMap<B, A> lambdaMapLike indexByCalculatedField_treeMultiMap(IF1<A, B> f, Iterable<A> l) {
  MultiMap<B, A> map = treeMultiMap();
  fOr (A a : l) {
    B val = f.get(a);
    if (val != null)
      map.put(val, a);
  }
  ret map;
}