static <A, B> Map<A, B> replaceNullValuesWith(Map<A, B> map, B value) {
  if (map == null) null;
  Map<A, B> m = similarEmptyMap(map);
  for (A a, B b : map) {
    m.put(a, or(b, value));
  }
  ret m;
}

static <A, B> Map<A, B> replaceNullValuesWith(B value, Map<A, B> map) {
  ret replaceNullValuesWith(map, value);
}