%%H Ueda <- Very good, especially 16(1) and 18(2)
%%H Please check this program through an experiment.

:-unknown(P,fail).

%23
is_concluded(contract(A,B,C),T):-
	become_effective(acceptance(B,A,offer(A,B,C)),T).
        
%15(1)
become_effective(offer(A,B,C),T1):-
	reach(offer(A,B,C),B,T1),
        \+(become_effective(withdrawl(A,B,offer(A,B,C)),B,T2)).

%15(2)
become_effective(withdrawl(A,B,offer(A,B,C)),B,T2):-
        reach(withdrawl(A,B,offer(A,B,C)),B,T2),
        \+((reach(offer(A,B,C),B,T1),T1@<T2)).

%16(1)
become_effective(revocation(A,B,offer(A,B,C)),B,T3):-
	\+(irrevocable(offer(A,B,C))),
        reach(revocation(A,B,offer(A,B,C)),B,T3),
	(\+((dispatch(acceptance(B,A,D),A,T4)));
	(dispatch(acceptance(B,A,D),A,T4),before(T3,T4))).

%18(2)
become_effective(acceptance(B,A,D),T):-
	become_effective(offer(A,B,C),T1),
	\+(become_effective(revocation(A,B,offer(A,B,C)),B,T3)),
	dispatch(acceptance(B,A,D),A,T4),
	reach(acceptance(B,A,D),A,T).

%case3d
reach(offer(anzai,bernard,sale(agriculture_machine,'$10000')),bernard,t0408).
reach(revocation(anzai,bernard,offer(anzai,bernard,sale(agriculture_machine,'$10000'))),bernard,t0409).
dispatch(acceptance(bernard,anzai,offer(anzai,bernard,sale(agriculture_machine,'$10000'))),anzai,t0410).
reach(acceptance(bernard,anzai,offer(anzai,bernard,sale(agriculture_machine,'$10000'))),anzai,t0410).