Validate that the UnaryTransformation is correctly representable.

def validate(self):
        """Validate that the UnaryTransformation is correctly representable."""
        _validate_operator_name(self.operator, UnaryTransformation.SUPPORTED_OPERATORS)

        if not isinstance(self.inner_expression, Expression):
            raise TypeError(u'Expected Expression inner_expression, got {} {}'.format(
                type(self.inner_expression).__name__, self.inner_expression))