code: def shift
        return nil if empty?
  
        if list_head.last?
          @_chain.tap { @_chain = nil }
        else
          old_head = list_head
          @_chain = list_head.next
          old_head.delete
        end
      end
docs: Shift the first item off the list.
  
   @return [Listable, nil] the first item in the list, or nil if the list is
     empty.