docs: def annotated_references(obj):
      """
      Return known information about references held by the given object.
  
      Returns a mapping from referents to lists of descriptions.  Note that there
      may be more than one edge leading to any particular referent; hence the
      need for a list.  Descriptions are currently strings.
  
      """
      references = KeyTransformDict(transform=id, default_factory=list)
      for type_ in type(obj).__mro__:
          if type_ in type_based_references:
              type_based_references[type_](obj, references)
  
      add_attr(obj, "__dict__", references)
      add_attr(obj, "__class__", references)
      if isinstance(obj, type):
          add_attr(obj, "__mro__", references)
  
      return references