'Checks if data is a key sequence
Returns True if yes, False if no, None if indeterminate.
If indeterminate, you should add more data.'

def check_key(self, data):

    r = self._kprefixes.get(data)
    if (r is True):
        return True
    if (r is False):
        return None
    if (r is None):
        return False
    raise RuntimeError()
