friend tb x y Steph (implication) rule label 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 function arg1 arg2 arg3 some other function arg4 arg5 arg6 rule (query) label rule label labelan example fact yet another relation constant e constant b role_name3role 3 rule (fact) label some constant d rulebase label labelan example rulebase