static <A> Collection<S> keysStartingWith2(S prefix, Map<S, A> map) {
  if (empty(prefix)) ret keys(map);
  if (map instanceof NavigableMap) {
    S lastKey = lexicographicallyNextString(prefix);
    ret keys(((NavigableMap) map).subMap(prefix, true, lastKey, false));
  }
  ret startingWith(prefix, keys(map));
}

static <A> Collection<S> keysStartingWith2(Map<S, A> map, S prefix) {
  ret keysStartingWith2(prefix, map);
}