code: func (tr Ranges) findFirstNotBefore(r Range) *list.Element {
  	if tr.sortedRanges == nil {
  		return nil
  	}
  	for e := tr.sortedRanges.Front(); e != nil; e = e.Next() {
  		if !e.Value.(Range).Before(r) {
  			return e
  		}
  	}
  	return nil
  }
docs: // findFirstNotBefore finds the first interval that's not before r.