def create_aln_expr(id, start=None, stop=None):
    """
    Create an alignment expression, such as ``n2[5:8]`` or ``tw1`` given an id, and start/stop range.

    :param id: ID with which to align
    :type id: str
    :param start: Range at which to start
    :type start: int
    :param stop: Range at which to stop
    :type stop: int
    """

    if start is None and stop is None:
        return id
    elif start is not None and stop is not None:
        return '%s[%d:%d]' % (id, start, stop)
    else:
        raise Exception('Invalid alignment expression request')