code: def lookup(instruction, instructions = None):
      
      if instructions is None:
          instructions = default_instructions
  
      if isinstance(instruction, str):
          return instructions[instruction]
      elif hasattr(instruction, "__call__"):
          rev = dict(((v,k) for (k,v) in instructions.items()))
          return rev[instruction]
      else:
          raise errors.MachineError(KeyError("Unknown instruction: %s" %
              str(instruction)))
docs: Looks up instruction, which can either be a function or a string.
      If it's a string, returns the corresponding method.
      If it's a function, returns the corresponding name.