我在序言中有以下程序。它是一个简单的问答系统，它收集事实并推断出新的结论。它：
- 通过询问动物的特征来发现动物（使用
recognition） - 描述知识库中任何动物的属性（使用
description）
程序中有一个错误，那就是它说 fact/2 是未定义的。此错误仅在recognition(animal)被询问时发生；description命令完美运行。
PS我知道它有一个非常简单的解决方案，但我找不到它是什么。
代码：
rule(1, animal, mammal, [c1]).
rule(2, animal, mammal, [c2]).
rule(3, animal, bird, [c3]).
rule(4, animal, bird, [c4, c5]).
rule(5, mammal, carnivore, [c6]).
rule(6, mammal, carnivore, [c7, c8, c9]).
rule(7, mammal, ungulate, [c10]).
rule(8, mammal, ungulate_toed, [c11]).
rule(9, carnivore, cheetah,[c12,c13]).
rule(10, carnivore, tiger, [c12,c14]).
rule(11, ungulate, giraffe, [c15, c16, c12,c13]).
rule(12, ungulate, zebra, [c17, c14]).
rule(13, bird, ostrich, [c18, c15, c16, c19]).
rule(14, bird, penguin,[c18, c20, c19]).
rule(15, bird, albatross, [c21]).
/* Recognition process : discover animal's name */
recognition(X):- rule(N, X, Y, Z),
discover(Z),
found(rule),
conclusion(X,Y,N),
recognition(Y),
retractall(fact(_,_)).
recognition(_):- retract(rule), write('Done.'),nl.
recognition(_):- write('Don"t know this animal.'),nl.
found(X):- X,!.
found(X):- assert(X).
/* Discovering process */
discover([]).
discover([X|Y]):- ask(X), discover(Y).
ask(X):- fact(X,Yes),!.
ask(X) :- fact(X, no),!, fail.
ask(c1):- write('has it hair?'), nl,!, complete(c1).
ask(c2):- write('does it give milk?'), nl,!, complete(c2).
ask(c3):- write('has it featherS?'), nl,!,complete(c3).
ask(c4):- write('does it fly?'), nl, !,complete(c4).
ask(c5):- write('does it lay eggs?'), nl, !, complete(c5).
ask(c6):- write('does it eat meat?'), nl, !, complete(c6).
ask(c7):- write('has it pointed teeth?'), nl, !, complete(c7).
ask(c8):- write('has it claws?'), nl, !, complete(c8).
ask(c9):- write('has it eyes pointing forward?'), nl,!, complete(c9).
ask(c10):- write('has it hoofs?'),nl, !, complete(c10).
ask(c11):- write('does it chew cud?'),nl,!, complete(c11).
ask(c12):- write('has it a tawny color?'),nl,!, complete(c12).
ask(c13):- write('has it dark spots?'),nl, !, complete(c13).
ask(c14):- write('has it black stripes?'), nl, !, complete(c14).
ask(c15):- write('has it long legs?'),nl,!, complete(c15).
ask(c16):- write('has it a long neck?'), nl, !, complete(c16).
ask(c17):- write('has it a white color?'), nl,!, complete(c17).
ask(c18):- write('does it not fly?'), nl, !, complete(c18).
ask(c19):- write('is it black and white?'),nl,!, complete(c19).
ask(c20):- write('does it swim?'),nl, !, complete(c20).
ask(c21):- write('is it a good flyer?'),nl,!, complete(c21).
complete(X):- read(Y), assert(fact(X,Y)), Y=yes.
/* Conclusion of the recognition process */
conclusion(X, Y, N):- nl, tab(4), write('--- the '), write(X),
write(' is a '), write(Y), write(' by rule '),
write(N), nl, nl.
/* Description process: discover animal's properties */
description(X):- rule(N, Y, X, Z), description1(Y,L,[]),
conclusion1(X, L, Y, Z, N).
description(_):- nl,write('Don''t know this animal.'),nl.
description1(Y, L, Ls):-rule(_, X, Y,_), description1(X, L, [X|Ls]) .
description1(_,L,L).
/* Conclusions of the description process */
conclusion1(X, L, Y, Z, N):- nl, write('a '),write(X),
write(' is an '),
output(L), write(Y),
write('satisfying conditions: '),nl,
output(Z), nl,write('by rule '),
write(N), write(' .') .
output([]).
output( [A|B] ) :- write(A), tab(1), output(B).
痕迹是：
[trace] 1 ?- recognition(animal).
Call: (6) recognition(animal) ? creep
Call: (7) rule(_G2040, animal, _G2042, _G2043) ? creep
Exit: (7) rule(1, animal, mammal, [c1]) ? creep
Call: (7) discover([c1]) ? creep
Call: (8) ask(c1) ? creep
Call: (9) fact(c1, yes) ? creep
Fail: (9) fact(c1, yes) ? creep
Redo: (8) ask(c1) ? creep
Call: (9) fact(c1, no) ? creep
Fail: (9) fact(c1, no) ? creep
Redo: (8) ask(c1) ? creep
Call: (9) write('has it hair?') ? creep
has it hair?
Exit: (9) write('has it hair?') ? creep
Call: (9) nl ? creep
Exit: (9) nl ? creep
Call: (9) complete(c1) ? creep
Call: (10) read(_G2043) ? creep
|: yes.
Exit: (10) read(yes) ? creep
^ Call: (10) assert(fact(c1, yes)) ? creep
^ Exit: (10) assert(fact(c1, yes)) ? creep
Call: (10) yes=yes ? creep
Exit: (10) yes=yes ? creep
Exit: (9) complete(c1) ? creep
Exit: (8) ask(c1) ? creep
Call: (8) discover([]) ? creep
Exit: (8) discover([]) ? creep
Exit: (7) discover([c1]) ? creep
Call: (7) found(rule) ? creep
Call: (8) rule ? creep
ERROR: found/1: Undefined procedure: rule/0
ERROR: However, there are definitions for:
ERROR: rule/4
Exception: (8) rule ?