// with some range in is2.
func (is Intervals) Overlaps(is2 Intervals) bool {
	// check for empty intervals
	if len(is) == 0 || len(is2) == 0 {
		return false
	}
	li := len(is)
	li2 := len(is2)
	// check for completely disjoint ranges
	if is[li-1].en <= is2[0].st ||
		is[0].st >= is2[li2-1].en {
		return false
	}