learn prolog now 2

 f(a).
f(b).
g(a).
g(b).
h(b).

k(X) :- f(X),g(X),h(X).

进入trace模式，

[trace] 2 ?- k(X).
Call: (6) k(_G348) ?
Call: (7) f(_G348) ?
Exit: (7) f(a) ?
Call: (7) g(a) ?
Exit: (7) g(a) ?
Call: (7) h(a) ?
Fail: (7) h(a) ?
Fail: (7) g(a) ?
Redo: (7) f(_G348) ?
Exit: (7) f(b) ?
Call: (7) g(b) ?
Exit: (7) g(b) ?
Call: (7) h(b) ?
Exit: (7) h(b) ?
Exit: (6) k(b) ?
X=b

重点是match和proof search