// does not store null values
static <B, A> MultiMap<B, A> lambdaMapLike treeMultiMapIndex(IF1<A, B> f, Iterable<A> l) {
  new TreeMultiMap<B, A> map;
  fOr (A a : l) {
    B val = f.get(a);
    if (val != null)
      map.put(val, a);
  }
  ret map;
}


static <B, A> MultiMap lambdaMapLike treeMultiMapIndex(Iterable<A> l, IF1<A, B> f) {
  ret treeMultiMapIndex(f, l);
}