def as_version(self, ver):
return self._generate_identity_name(ver)<EOL>
Return an edited name, with a different version number or no version

        :param ver: A Version number for the returned document. May also be an integer prefixed with '+'
        ( '+1' ) to set the version ahead of this document's version. Prefix with a '-' to set a version behind.

        Typical use of the version math feature is to get the name of a document one version behind:

        >>> doc.as_version('-1')