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