def _run_formula(self, variable, population, period):
        """
            Find the ``variable`` formula for the given ``period`` if it exists, and apply it to ``population``.
        """

        formula = variable.get_formula(period)
        if formula is None:
            return None

        if self.trace:
            parameters_at = self.trace_parameters_at_instant
        else:
            parameters_at = self.tax_benefit_system.get_parameters_at_instant

        if formula.__code__.co_argcount == 2:
            array = formula(population, period)
        else:
            array = formula(population, period, parameters_at)

        self._check_formula_result(array, variable, population, period)
        return array
Find the ``variable`` formula for the given ``period`` if it exists, and apply it to ``population``.