new_contents: 
  class Trigger:
      def __init__(self):
          pass
  
      def __call__(self, step):
          raise NotImplementedError
  
  
  class PeriodicTrigger(Trigger):
      def __init__(self, period, phase=0):
          self.period = period
          self.phase = phase
  
      def __call__(self, timestep):
          return (timestep - self.phase) % self.period == 0
  
subject: Add triggers for scheduling events