docs: def base_concrete_model(abstract, model):
      """
      Used in methods of abstract models to find the super-most concrete
      (non abstract) model in the inheritance chain that inherits from the
      given abstract model. This is so the methods in the abstract model can
      query data consistently across the correct concrete model.
  
      Consider the following::
  
          class Abstract(models.Model)
  
              class Meta:
                  abstract = True
  
              def concrete(self):
                  return base_concrete_model(Abstract, self)
  
          class Super(Abstract):
              pass
  
          class Sub(Super):
              pass
  
          sub = Sub.objects.create()
          sub.concrete() # returns Super
  
      In actual yacms usage, this allows methods in the ``Displayable`` and
      ``Orderable`` abstract models to access the ``Page`` instance when
      instances of custom content types, (eg: models that inherit from ``Page``)
      need to query the ``Page`` model to determine correct values for ``slug``
      and ``_order`` which are only relevant in the context of the ``Page``
      model and not the model of the custom content type.
      """
      if hasattr(model, 'objects'):
          # "model" is a model class
          return (model if model._meta.abstract else
                  _base_concrete_model(abstract, model))
      # "model" is a model instance
      return (
          _base_concrete_model(abstract, model.__class__) or
          model.__class__)