def sync(self, old_token=None):
<EOL>token = "<STR_LIT>" % self.etag.strip("<STR_LIT>")<EOL>if old_token:<EOL><INDENT>raise ValueError("<STR_LIT>")<EOL><DEDENT>return token, self.list()<EOL>
Get the current sync token and changed items for synchronization.

        ``old_token`` an old sync token which is used as the base of the
        delta update. If sync token is missing, all items are returned.
        ValueError is raised for invalid or old tokens.