def get_variable(self, variable_name, check_existence = False):
        """
        Get a variable from the tax and benefit system.

        :param variable_name: Name of the requested variable.
        :param check_existence: If True, raise an error if the requested variable does not exist.
        """
        variables = self.variables
        found = variables.get(variable_name)
        if not found and check_existence:
            raise VariableNotFound(variable_name, self)
        return found
Get a variable from the tax and benefit system.

        :param variable_name: Name of the requested variable.
        :param check_existence: If True, raise an error if the requested variable does not exist.