def insert index, data
      old_node = @first 
      index.times do
        old_node = old_node.next
      end
      new_node = LLNode.new(data, old_node, old_node.previous)
      old_node.previous.next = new_node
      old_node.previous = new_node
      self
    end
def insert index, data
      old_node = @first 
      index.times do
        old_node = old_node.next
      end
      new_node = LLNode.new(data, old_node, old_node.previous)
      old_node.previous.next = new_node
      old_node.previous = new_node
      self
    end
Insert a node with +:data+ at +:index+.
 All nodes +:index+ and above get moved along one.