static <A, B, C> DoubleKeyedMap<A, B, C> pairKeyMapToDoubleKeyedMap(Map<Pair<A, B>, C> map) {
  new DoubleKeyedMap<A, B, C> out;
  fOr (Pair<A, B> p, C c : map)
    out.put(p, c);
  ret out;
}