You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: ast.rb
    path of file: ./repos/writing-a-compiler-in-ruby
    
    the code of the file until where you have to start completion: 
module AST

  # Adds properties to AST nodes that simplify error
  # reporting, debugging etc.
  #
  # This will later also provide a location 
    
    def self.[](*args)
      # FIXME: Code generation bug causes
      # pos not to get initialized if the if condition below is not satisfied.
      pos = nil
      if args.size > 0 and args.first.is_a?(Scanner::Position) || args.first.nil?
        pos = args.shift
      end