Wrong result in append with appended tail element

[LhKipp]

Code:

The output of SWI-Prolog for the query: append([a], [b|Xs], Zs). is:
Zs = [a, b|Xs].

The output of Prolog_BFS is:
[Zs / [a,b|X]] [Xs / Xs]

[Xs / Xs] should not be shown. Furthermore it is questionable why |X is shown and not |Xs.

If possible, the output of Prolog_BFS should be the same as the output of SWI-Prolog.