static MultiMap mapMultiMapValues(O func, MultiMap mm) {
  MultiMap m = similarEmptyMultiMap(mm);
  for (O key : keys(mm))
    for (O value : mm.get(key))
      m.put(key, callF(func, value));
  ret m;
}

static MultiMap mapMultiMapValues(MultiMap mm, O func) {
  ret mapMultiMapValues(func, mm);
}

static <A, B, C> MultiMap<A, C> lambdaMapLike mapMultiMapValues(IF1<B, C> func, MultiMap<A, B> mm) {
  ret mapMultiMapValues((O) func, mm);
}

Began life as a copy of #1004652