The toplevel supports two modes for reporting that it is undefined
whether the current answer is true. The mode is selected by the Prolog
true, the toplevel uses call_delays/2
to find the conditional answers used and the residual program
associated with these answers. It then prints the residual program,
followed by the answer and the conditional answers. For undefined/0,
this results in the following output:
?- undefined. % WFS residual program undefined :- tnot(undefined). undefined.
:- table p/0, q/0. p :- tnot(q). q :- tnot(p).
?- p. % WFS residual program p :- tnot(q). q :- tnot(p). p. ?- set_prolog_flag(toplevel_list_wfs_residual_program, false). true. ?- p. undefined.