为什么这两个prolog代码会给出不同的结果？

Question

call((append(As,Bs,[1,2,3,4]),writeq(user,append(As,Bs,[1,2,3,4])))).

结果：

append([],[1,2,3,4],[1,2,3,4])
As = [],
Bs = [1,2,3,4] ? ;
append([1],[2,3,4],[1,2,3,4])
As = [1],
Bs = [2,3,4] ? ;
append([1,2],[3,4],[1,2,3,4])
As = [1,2],
Bs = [3,4] ? ;
append([1,2,3],[4],[1,2,3,4])
As = [1,2,3],
Bs = [4] ? ;
append([1,2,3,4],[],[1,2,3,4])
As = [1,2,3,4],
Bs = [] ? ;
no

但如果我试试这个：

 call((append(As,Bs,[1,2,3,4]) -> writeq(append(As,Bs,[1,2,3,4])) ; writeq('There are no solutions.\n'))).

它回馈了这个：

append([],[1,2,3,4],[1,2,3,4])
As = [],
Bs = [1,2,3,4] ? ;
no

有什么区别？我尝试写它，以便它可以判断是否有0个解决方案。