Returns the name of pvariable.

        Returns:
            Name of pvariable.

        Raises:
            ValueError: If not a pvariable expression.

def name(self) -> str:
        '''Returns the name of pvariable.

        Returns:
            Name of pvariable.

        Raises:
            ValueError: If not a pvariable expression.
        '''
        if not self.is_pvariable_expression():
            raise ValueError('Expression is not a pvariable.')
        return self._pvar_to_name(self.args)