old_contents: #!/usr/bin/env python
  
  from nodes import Node
  
  class HeadEnd(Node):
      char = "}"
      args = 1
      results = 2
      def func(self, inp:Node.indexable):
          return [inp[0], inp[-1]]
new_contents: #!/usr/bin/env python
  
  from nodes import Node
  
  class HeadEnd(Node):
      char = "}"
      args = 1
      results = 2
      
      def double(self, inp: Node.number):
          return inp*2
      
      
      def head_end(self, inp:Node.indexable):
          return [inp[0], inp[-1]]
subject: Add double function to head_end node