code: func (s *SkipList) SeekToLast() Iterator {
  	current := s.footer
  	if current == nil {
  		return nil
  	}
  
  	return &iter{
  		current: current,
  		key:     current.key,
  		list:    s,
  		value:   current.value,
  	}
  }
docs: // SeekToLast returns a bidirectional iterator starting from the last element
  // in the list if the list is populated; otherwise, a nil iterator is returned.