def get_lids(f):
    """get identifiers that specify an absolute location (i.e. start with '/')"""
    lids={}
    for ns in f.ddef.keys():
        lids[ns] = []
        structures = f.ddef[ns]['structures']
        for id in structures:
            if id[0] == '/':
                lids[ns].append(id)
    return lids