docs: def label_from_instance(self, obj):
          """
          Creates labels which represent the tree level of each node when
          generating option labels.
          """
          level = getattr(obj, obj._mptt_meta.level_attr)
          level_indicator = mark_safe(conditional_escape(self.level_indicator) * level)
          return mark_safe(u'%s %s' % (level_indicator, conditional_escape(smart_unicode(obj))))