The toplevel supports two modes for reporting that it is undefined
whether the current answer is true. The mode is selected by the Prolog
flag
toplevel_list_wfs_residual_program.
If true, the toplevel uses call_delays/2
and delays_residual_program/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.
If the toplevel_list_wfs_residual_program is false, any undefined answer is a conjunction with undefined/0. See the program and output below.
:- 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.