writeq/1 incorrect writing of list

[UWN]

?- append(Xs, Ys, [a,b|Xs]), Ys = [b,a], writeq(Ys), nl.
   outputs("[b]\n"), unexpected,
   Xs = "a", Ys = "ba"
;  ... .

[UWN]

Shorter:

r(Ys) :-
   [a,b|Xs] = [X|Ys],
   Xs = [X].


?- r(Ys), writeq(Ys), nl.
[b]
   Ys = "ba".

[UWN]

r(Ys) :-
   [a,b|Xs] = [_|Ys],
   Xs = [e].

?- r(Ys), writeq(Ys), nl.
[b]   % unexpected
   Ys = "be".