code: def argval(self):
          
          if self.arg is None or any(x is None for x in self.arg):
              return None
  
          for x in self.arg:
              if not isinstance(x, int):
                  raise InvalidArgError(self.arg)
  
          return self.arg
docs: Returns the value of the arg (if any) or None.
          If the arg. is not an integer, an error be triggered.