code: func (s *SkipList) SeekToFirst() Iterator {
  	if s.length == 0 {
  		return nil
  	}
  
  	current := s.header.next()
  
  	return &iter{
  		current: current,
  		key:     current.key,
  		list:    s,
  		value:   current.value,
  	}
  }
docs: // SeekToFirst returns a bidirectional iterator starting from the first element
  // in the list if the list is populated; otherwise, a nil iterator is returned.