static IntPairTreeSet intPairTreeSetWithComparator(Comparator<IntPair> comparator) {
  ret new IntPairTreeSet {
    @Override public int compare(long a, long b) {
      ret comparator.compare(longToIntPair(a), longToIntPair(b));
    }
  };
}

ifclass LongComparator
static IntPairTreeSet intPairTreeSetWithComparator(LongComparator comparator) {
  ret new IntPairTreeSet {
    @Override public int compare(long a, long b) {
      ret comparator.compare(a, b);
    }
  };
}
endif

Began life as a copy of #1030408