static <A, B, C, D> Map<C, D> mapMapToMap(IF2<A, B, Pair<C, D>> f, Map<A, B> m) {
  if (m == null) null;
  Map<C, D> map = similarEmptyMap(m);
  for (Map.Entry<A, B> e : m.entrySet()) {
    Pair<C, D> p = f.get(e.getKey(), e.getValue());
    map.put(p.a, p.b);
  }
  ret map;
}

static <A, B, C, D> Map<C, D> mapMapToMap(Map<A, B> m, IF2<A, B, Pair<C, D>> f) {
  ret mapMapToMap(f, m);
}

Began life as a copy of #1024294