def delete index
      current = @first
      index.times do
        current = current.next
      end
      current.previous.next = current.next
      current.next.previous = current.previous
    end
def delete index
      current = @first
      index.times do
        current = current.next
      end
      current.previous.next = current.next
      current.next.previous = current.previous
    end
Delete the node at +:index+