4.12.7.2 Example: retract

| ?- assert(p(1)), assert(p(2)), assert(p(3)).

yes
| ?- p(N), writeq(N), nl, retract(p(2)),
     retract(p(3)), fail.
1
2
3

no
| ?- p(N), writeq(N), fail.
1
no
| ?- 

At the first call to p/1, the procedure has three clauses. These remain visible throughout execution of the call to p/1. Thus, when backtracking is forced by fail/0, N is bound to 2 and written. The retraction is again attempted, causing backtracking into p/1. N is bound to 3 and written out. The call to retract/1 fails. There are no more clauses in p/1, so the query finally fails. A subsequent call to p/1, made after the retractions, sees only one clause.