ORA-01436: CONNECT BY loop in user data

drop table tq84_edge;
drop table tq84_node;

create table tq84_node (
   id   number primary key,
   name varchar2(10)
);

create table tq84_edge (
   node_from not null references tq84_node,
   node_to   not null references tq84_node
);

insert into tq84_node values (1, 'n1');
insert into tq84_node values (2, 'n2');
insert into tq84_node values (3, 'n3');
insert into tq84_node values (4, 'n4');
insert into tq84_node values (5, 'n5');

insert into tq84_edge values (3, 2);
insert into tq84_edge values (2, 4);
insert into tq84_edge values (2, 1);
insert into tq84_edge values (1, 5);

--
-- uh oh:
--   the following record will cause
--   ORA-01436: CONNECT BY loop in user data
--
insert into tq84_edge values (1, 3);

select
   level,
   nf.name    node_from,
   nt.name    node_to
from
   tq84_edge   ed                          join
   tq84_node   nf on nf.id = ed.node_from  join
   tq84_node   nt on nt.id = ed.node_to
start with
   ed.node_from =  3
connect by
   prior ed.node_to = ed.node_from
;

alter session set "_dump_connect_by_loop_data" = true;

CONNECT BY loop in user data :  (2) --> (1) --> (3) --> (2)