new_contents: #!/usr/bin/env python
  
  import lang_ast
  from nodes import Node
  
  class IfKill(Node):
      char = ".I"
      args = 1
      results = None
      
      def __init__(self, ast: Node.EvalLiteral):
          self.ast = ast
          if self.ast.nodes == []:
              self.ast.add_node("\n")
          
      def prepare(self, stack):
          self.args = max(len(stack), 1)
              
      @Node.test_func([5,1], [10], "}")
      @Node.test_func([5,0], [], "}")
      def func(self, *args):
          """Takes stack.
  if arg1: stack = eval_literal(stack[:-1]) (extend mode)
  else: stack = []"""
          args = list(args)
          if args[-1]:
              args = args[:-1]
              stack = self.ast.run(args)
              return stack
          return []
      
      def __repr__(self):
          return "%s: %r"%(self.__class__.__name__, self.ast)
          
subject: Add if_kill node where kills stack if false