def auto_generate_child_rel(self, klass, param):
        """
        Auto-populate self.child_rel1 and self.child_rel2.
        This requires self.down to be another valid DiffLevel object.
        :param klass: A ChildRelationship subclass describing the kind of parent-child relationship,
                      e.g. DictRelationship.
        :param param: A ChildRelationship subclass-dependent parameter describing how to get from parent to child,
                      e.g. the key in a dict
        """
        if self.down.t1 is not notpresent:
            self.t1_child_rel = ChildRelationship.create(
                klass=klass, parent=self.t1, child=self.down.t1, param=param)
        if self.down.t2 is not notpresent:
            self.t2_child_rel = ChildRelationship.create(
                klass=klass, parent=self.t2, child=self.down.t2, param=param)
Auto-populate self.child_rel1 and self.child_rel2.
        This requires self.down to be another valid DiffLevel object.
        :param klass: A ChildRelationship subclass describing the kind of parent-child relationship,
                      e.g. DictRelationship.
        :param param: A ChildRelationship subclass-dependent parameter describing how to get from parent to child,
                      e.g. the key in a dict