rule label labelan example fact friend tb x y Steph friend tb x y Jill friend Jane Joan Jack y Frank yar constructor z yar2c some relation role_name1role 1 constant a constant b role_name2role 2 another relation constant c constant d yac constant g constant h empty constant h some relation role_name1role 1 constant a constant b constant b b role_name2role 2 another relation constant c constant d x role_name3role 3 test yet another relation constant b constant e constant b role_name3role 3 b