Return an instantiated Expression as
        ``cls.create(*self.args, **self.kwargs)``

        Args:
            cls (class): The class of the instantiated expression. If not
            given, ``self.cls`` will be used.

def instantiate(self, cls=None):
        """Return an instantiated Expression as
        ``cls.create(*self.args, **self.kwargs)``

        Args:
            cls (class): The class of the instantiated expression. If not
            given, ``self.cls`` will be used.
        """
        if cls is None:
            cls = self.cls
        if cls is None:
            raise TypeError("cls must a class")
        return cls.create(*self.args, **self.kwargs)