%IR

| ?- ['contract(3d).txt'].
% consulting h:/contract(3d).txt...
% Undefined predicates will just fail (fail)
* [T2] - singleton variables in user:become_effective/2
* Approximate lines: 4-8, file: 'h:/contract(3d).txt'
* [C,T1] - singleton variables in user:become_effective/2
* Approximate lines: 12-18, file: 'h:/contract(3d).txt'
% consulted h:/contract(3d).txt in module user, 0 msec 2344 bytes
yes
| ?-
| ?-
| ?- listing.
become_effective(offer(A,B,C), D) :-
        reach(offer(A,B,C), B, D),
        \+become_effective(withdrawl(A,B,offer(A,B,C)),_).
become_effective(withdrawl(A,B,offer(A,B,C)), D) :-
        reach(withdrawl(A,B,offer(A,B,C)), B, D),
        \+ (reach(offer(A,B,C),B,E),before(E,D)).
become_effective(acceptance(A,B,C), D) :-
        reach(acceptance(A,B,C), B, D),
        become_effective(offer(B,A,_), _).

before(A, B) :-
        A@<B.

is_concluded(contract(A,B,C), D) :-
        become_effective(acceptance(B,A,offer(A,B,C)), D).

reach(offer(anzai,bernard,sale(agriculture_machine,'$50000')), bernard, t0408
reach(acceptance(bernard,anzai,offer(anzai,bernard,sale(agriculture_machine,'
000'))), anzai, t0410).
reach(withdrawl(anzai,bernard,offer(anzai,bernard,sale(agriculture_machine,'$
00'))), bernard, t0409).

yes
| ?- trace.
% The debugger will first creep -- showing everything (trace)
yes
% trace
| ?- is_concluded(X,T).
        1      1 Call: is_concluded(_463,_483) ?
        2      2 Call: become_effective(acceptance(_1106,_1105,offer(_1105,_1
,_1107)),_483) ?
        3      3 Call: reach(acceptance(_1106,_1105,offer(_1105,_1106,_1107))
105,_483) ?
        3      3 Exit: reach(acceptance(bernard,anzai,offer(anzai,bernard,sal
griculture_machine,'$50000'))),anzai,t0410) ?
        4      3 Call: become_effective(offer(anzai,bernard,_1754),_1757) ?
        5      4 Call: reach(offer(anzai,bernard,_1754),bernard,_1757) ?
        5      4 Exit: reach(offer(anzai,bernard,sale(agriculture_machine,'$5
0')),bernard,t0408) ?
        6      4 Call: become_effective(withdrawl(anzai,bernard,offer(anzai,b
ard,sale(agriculture_machine,'$50000'))),_3781) ?
        7      5 Call: reach(withdrawl(anzai,bernard,offer(anzai,bernard,sale
riculture_machine,'$50000'))),bernard,_3781) ?
        7      5 Exit: reach(withdrawl(anzai,bernard,offer(anzai,bernard,sale
riculture_machine,'$50000'))),bernard,t0409) ?
        8      5 Call: reach(offer(anzai,bernard,sale(agriculture_machine,'$5
0')),bernard,_5843) ?
        8      5 Exit: reach(offer(anzai,bernard,sale(agriculture_machine,'$5
0')),bernard,t0408) ?
        9      5 Call: before(t0408,t0409) ?
       10      6 Call: t0408@<t0409 ?
       10      6 Exit: t0408@<t0409 ?
        9      5 Exit: before(t0408,t0409) ?
        6      4 Fail: become_effective(withdrawl(anzai,bernard,offer(anzai,b
ard,sale(agriculture_machine,'$50000'))),_3781) ?
        4      3 Exit: become_effective(offer(anzai,bernard,sale(agriculture_
hine,'$50000')),t0408) ?
        2      2 Exit: become_effective(acceptance(bernard,anzai,offer(anzai,
nard,sale(agriculture_machine,'$50000'))),t0410) ?
        1      1 Exit: is_concluded(contract(anzai,bernard,sale(agriculture_m
ine,'$50000')),t0410) ?
T = t0410,
X = contract(anzai,bernard,sale(agriculture_machine,'$50000')) ?
yes