A versioned dictionary class derived from the native dict class.

Hint: Only runs with Python3!

This class is useful when subsequent versions of a dictionary frequently have common key-value pairs which consume quite some memory or would be time-consuming to deep-copy for each new version.

When 'archiving' a version, only the difference relative to the last stored version is put on the archive stack (kind of incremental backup): Added keys and their values, deleted keys and their values and modified keys and their values are are stored.

Limitations::

• If rewinding of versions happens very often, this approach lacks performance, in particular when there are many versions.
• When archiving a version, the difference is examined only on the highest level: In case of a common key the substructure of the values is not examined. (That would somewhat correspond to a differential backup.)
• When rewinding or looking up a specific archived version, the item ordering is usually not preserved (as is not to be expected from a :class:`dict`).
• keys are assumed to objects of type :class:`str`. We do deep-copy keys when archiving, but this is not tested.

Example use case: Version 0 of the dictionary has just one key containing the fulltext of a longer document. Other versions add or delete keys containing intermediate results in analyzing the document, which may themselves be of considerable size, but which don't change the document.

There is no documentation beyond the unit tests.