static <A, B, C> MultiMap<B, C> lambdaMapLike mapMultiMapKeys(IF1<A, C> f, MultiMap<A, B> mm) {
  MultiMap m = similarEmptyMultiMap(mm);
  for (key : keys(mm))
    m.put(f.get(key), mm.get(key));
  ret m;
}

static <A, B, C> MultiMap<B, C> lambdaMapLike mapMultiMapKeys(MultiMap<A, B> mm, IF1<A, C> f) {
  ret mapMultiMapKeys(f, mm);
}