The implementation of this one is weird. By the time
        the {'$exists': True} spec gets to the dispatched
        handler, the key presumably exists.

        So we just parrot the assertion the spec makes. If it
        asserts the key exists, we return True. If it asserts
        the key doesn't exist, we return False, because that
        can't be true.

def handle_exists(self, spec, checkable):
        '''The implementation of this one is weird. By the time
        the {'$exists': True} spec gets to the dispatched
        handler, the key presumably exists.

        So we just parrot the assertion the spec makes. If it
        asserts the key exists, we return True. If it asserts
        the key doesn't exist, we return False, because that
        can't be true.
        '''
        if not isinstance(spec, bool):
            msg = 'The argument of an exists query must be of type bool.'
            raise InvalidQuery(msg)
        return spec