code: def record_calculation_start(self, variable_name, period, **parameters):
          
          key = self._get_key(variable_name, period, **parameters)
  
          if self.stack:  # The variable is a dependency of another variable
              parent = self.stack[-1]
              self.trace[parent]['dependencies'].append(key)
          else:  # The variable has been requested by the client
              self.requested_calculations.add(key)
  
          if not self.trace.get(key):
              self.trace[key] = {'dependencies': [], 'parameters': {}}
  
          self.stack.append(key)
          self._computation_log.append((key, len(self.stack)))
          self.usage_stats[variable_name]['nb_requests'] += 1
docs: Record that OpenFisca started computing a variable.
  
              :param str variable_name: Name of the variable starting to be computed
              :param Period period: Period for which the variable is being computed
              :param list parameters: Parameter with which the variable is being computed