def params(self):
        """
        Raises an ``AttributeError``if the definition is not callable.
        Otherwise returns a list of `ValueElement` that represents the params.
        """
        if self.context.el_type in [Function, Subroutine]:
            return self.evaluator.element.parameters
Raises an ``AttributeError``if the definition is not callable.
        Otherwise returns a list of `ValueElement` that represents the params.