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

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