def trigger_paths(tnum):
    """
    Return components of the path to the data for trigger `tnum`, both at
    the COSSC FTP site and locally:  (group, trigger, remote).

    group       name of the group directory containing the trigger directory
    trigger     name of the trigger directory, "<tnum>_burst"
    remote      tail of the ftp path @ COSSC, "<group>/<trigger>"
    """
    # Trigger data is in groups spanning a trigger range of 200.
    fac = tnum//200
    l, u = fac*200+1, (fac+1)*200
    group = '%05d_%05d' % (l, u)
    trigger = '%05d_burst' % tnum
    remote = group + '/' + trigger + '/'  # don't use join; not a local path
    return group, trigger, remote