static <A, B> Set<A> sortedKeys(Map<A, B> map) {
  ret sorted(keys(map));
}

static Set sortedKeys(O map) {
  ret sorted(keys(map));
}

ifclass MultiSet
  static <A> Set<A> sortedKeys(MultiSet<A> ms) {
    ret sorted(keys(ms));
  }
endif

ifclass MultiMap
  static <A, B> Set<A> sortedKeys(MultiMap<A, B> mm) {
    ret sorted(keys(mm));
  }
endif