static IntRange intersectIntRanges(IntRange a, IntRange b) {
  int start = max(a.start, b.start);
  int end = min(a.end, b.end);
  ret start <= end ? IntRange(start, end) : null;
}

static L<IntRange> intersectIntRanges(Iterable<IntRange> l, IntRange b) {
  ret map(l, r -> intersectIntRanges(r, b));
}