static <A, B, C> Map<A, Map<B, C>> pairKeysToMapOfMap(Map<Pair<A, B>, C> map) {
  if (map == null) null;
  new Map<A, Map<B, C>> out;
  for (Pair<A, B> key, C value : map)
    mapGetOrCreate(out, key.a, HashMap.class).put(key.b, value);
  ret out;
}