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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def parse_secondary_types(self, request):
 """ Return secondary event types that occurrences must belong to, or `None` if there is no constraint on secondary type. """

        if request.GET.getlist('secondary_types'):
            return EventType.objects.filter(
                slug__in=request.GET.getlist('secondary_types'))
        return None