def _join_mgm_lfn(mgm, lfn):
    """
    Joins mgm and lfn, ensures correct formatting.
    Will throw an exception of the lfn does not start with '/store'
    """
    if not lfn.startswith('/store'):
        raise ValueError(
            "This function expects filenames that start with '/store'")
    if not mgm.endswith('/'):
        mgm += '/'
    return mgm + lfn
