code: func (it *RangeIter) Next() bool {
  	if it.ranges == nil {
  		return false
  	}
  	if it.cur == nil {
  		it.cur = it.ranges.Front()
  	} else {
  		it.cur = it.cur.Next()
  	}
  	return it.cur != nil
  }
docs: // Next moves to the next item.