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

%%H 最後の行の要件は必要ですか？

%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)),T2)).
%15(2)
become_effective(withdrawl(A,B,offer(A,B,C)),T2):-
        reach(withdrawl(A,B,offer(A,B,C)),B,T2),
        \+(reach(offer(A,B,C),B,T1),T1@<T2).
%18(2)
become_effective(acceptance(B,A,C),T):-
	reach(acceptance(B,A,C),A,T).
%case3c
reach(offer(anzai,bernard,sale(agriculture_machine,'$50000')),bernard,t0408).
reach(withdrawl(anzai,bernard,offer(anzai,bernard,sale(agriculture_machine,'$50000'))),t0407).
reach(acceptance(bernard,anzai,offer(anzai,bernard,sale(agriculture_machine,'$50000'))),anzai,t0409).