ask(Attribute,Value):-
  current_virus(X),
  known(yes,Attribute,Value,_),     % succeed if we know its true
  retractall(known(yes,Attribute,Value,_)),
  assert(known(yes,Attribute,Value,X)),
  !.                              % and dont look any further

ask(Attribute,Value):-
  current_virus(X),
  known(no,Attribute,Value,_),       % fail if we know its false
  retractall(known(no,Attribute,Value,_)),
  assert(known(no,Attribute,Value,X)),
  !, fail.                        % and dont look any further

ask(A,V):-
  write(A:V),   % if we get here, we need to ask.
  write('? : '),
  read(Answer),
  assert(known(Answer,A,V,_)),
  Answer = yes.