code: def _valid_comparison(time_a, time_b, event_a, event_b):
      
      if time_a == time_b:
          # Ties are only informative if exactly one event happened
          return event_a != event_b
      if event_a and event_b:
          return True
      if event_a and time_a < time_b:
          return True
      if event_b and time_b < time_a:
          return True
      return False
docs: True if times can be compared.