'Add the checker specified.  If any checkers are added, the default
checkers are automatically cleared and the only checkers will be the
specified one(s).'

def addChecker(self, checker):

    if self._usingDefaultAuth:
        self['credCheckers'] = []
        self['credInterfaces'] = {}
        self._usingDefaultAuth = False
    super(Options, self).addChecker(checker)
