new_contents: class Identifier:
      def __init__(self, id):
          self.id = id
          
      def eval(self, env):
          if self.id in env:
              return env[self.id]
          else:
              error("Referencing " + self.id + " before assignment")
          
      def __repr__(self):
          return "Identifier: {0}".format(self.id)
  
subject: Create identifier.py