meta-for MultiMap also as MultiSetMap {
  static <A, B> LPair<A, B> multiMapSingletonPairs(MultiMap<A, B> mm) {
    new LPair<A, B> out;
    if (mm != null) for (A key : mm.keySet()) {
      var values = mm.get(key);
      if (l(values) == 1)
        out.add(pair(key, first(values)));
    }
    ret out;
  }
}