'Unregisters the given class.
If the specified class isn\'t registered, ``NotRegistered`` will
be raised.'

def _unregister(self, cls):

    if (not issubclass(cls, self._registerable_class)):
        raise ValueError(('Only    %s    classes    or    subclasses    may    be    unregistered.' % self._registerable_class))
    if (cls not in self._registry.keys()):
        raise NotRegistered(('%s    is    not    registered' % cls))
    del self._registry[cls]
    return True
