Some simple tools to enable undoable histories in asyntactically unobtrusive way.

(+) in general, a tracked object will add change-events to a history object
(+) format of these events type- (& implementation-) specifc, class defines
(+) a function to call to 'undo' the changes
(+) we subclass various standard types (list , set, dict, ...? ) to provide these functions
(+) each history chunk is a dictionary, with
  (-) keys - objects that changed
  (-) values - objects describing the totality of changes
    to the changed objects, in a format appropriate to
    pass to the changed objects undo() or redo() methods