code: def __initial_minus_queryset(self):
          
          queryset = self.get_queryset()
  
          def initial_not_in_queryset(initial):
              for x in queryset:
                  if x.slot == initial['slot']:
                      return False
  
              return True
  
          return list(filter(initial_not_in_queryset, self._initial))
docs: Gives all elements from self._initial having a slot value that is not already
          in self.get_queryset()