code: def transition(self, index, event_time):
          
          for state, affected in self._get_state_pops(index):
              if not affected.empty:
                  state.next_state(affected.index, event_time, self.population_view.subview([self.state_column]))
docs: Finds the population in each state and moves them to the next state.
  
          Parameters
          ----------
          index : iterable of ints
              An iterable of integer labels for the simulants.
          event_time : pandas.Timestamp
              The time at which this transition occurs.