Given a current recorder and a token index, try to find a recorder.

def _find_recorder(recorder, tokens, index):
    """Given a current recorder and a token index, try to find a recorder."""
    if recorder is None:
        # See if we can start recording something
        for recorder_factory in _RECORDERS:
            recorder = recorder_factory.maybe_start_recording(tokens,
                                                              index)
            if recorder is not None:
                return recorder

    return recorder