Append/3 isn't deterministic if first arg is var?

Help!
8

As I understand it, the only slight inelegance with swi-prolog’s append/3 currently is the unwanted choicepoint when args 2 & 3 are lists, and arg 1 is var. This solves that issue, with a reasonable performance hit:

append2a(Start, End, Both) :-
    % Guard against unwanted choicepoint
    is_list(Both),
    is_list(End),
    !,
    append(Start, End, Both),
    !.
append2a(Start, End, Both) :-
    append(Start, End, Both).

I don’t see a need to check the first arg.

Edit: Tweaked to check Both then End vars, which seems like a slightly more sensible order, for performance/likelihood.

This is basically what jan already wrote above. I don’t see a better way.