Like elephants don't forget anything, so does not
collective.elephantvocabulary. It provides a wrapper around for existing
zope.schema vocabularies and make them not forget anything.
Example usecase would be a vocabulary (source) of users which from certain
point in time wants to hide / deactivate some users for form or listing. But
at the same time you want keep old references to user term working. This is
where collective.elephantvocabulary comes into the picture. With it you
wrap existing vocabulary of users and provide set of hidden list of users
(term values).