def flow_direction(arbor0, arbor1):
    """
    Return direction of information flow between arbors.

    Returns
    -------
    result : set
        {'r'} for arbor0 -> arbor1;
        {'l'} for arbor0 <- arbor1;
        {'l', 'r'} for arbor0 <-> arbor1;
        set() for no flow.
    """

    result = set()
    if 'b' in arbor0['neurite'] and 's' in arbor1['neurite']:
        result.add('r')
    if 's' in arbor0['neurite'] and 'b' in arbor1['neurite']:
        result.add('l')
    return result