@property<EOL><INDENT>def changed(self):<DEDENT>
return {key: self._instance[key] for key in self._previous}<EOL>
Returns a dict containing just the fields which have changed on this
Document since it was created or last saved, together with their new
values.

    doc['name']             # => 'bob'
    doc['name'] = 'clive'
    doc.changed             # => {'name': 'clive'}