ask(A, V):-
known(yes, A, V), % succeed if true
!. % stop looking

ask(A, V):-
known(_, A, V), % fail if false
!, fail.

ask(A, V):-
write(A:V), % ask user
write('? : '),
read(Y), % get the answer
asserta(known(Y, A, V)), % remember it
Y == yes. % succeed or fail