docs: def generate_identifier(sender, instance, **kwargs):
      """
      Generate and set identifier of concept before saving object to DB
  
      Args:
          sender (class): should be Concept
          instance (Concept): saving concept
      """
      identifier = Concept.create_identifier(instance.query)
      qs = Concept.objects.filter(identifier=identifier, lang=instance.lang)
      if instance.pk:
          qs = qs.exclude(pk=instance.pk)
      if qs.count() > 0:
          raise ValueError("Concept identifier conflict")
      instance.identifier = identifier