code: def validate_items(all_items):
      
      validation = []
      for item in all_items:
          if item.talk is not None and item.page is not None:
              validation.append(item)
          elif item.talk is None and item.page is None:
              validation.append(item)
          elif item.talk and item.talk.status not in [ACCEPTED, CANCELLED]:
              validation.append(item)
      return validation
docs: Find errors in the schedule. Check for:
           - pending / rejected talks in the schedule
           - items with both talks and pages assigned
           - items with neither talks nor pages assigned