This goes off into infinity with:

```
?- split_list_at_nth1(N, [a,b,c], [a,b,c|T], End).
N = 3,
T = End, End = [] ;
ERROR: Stack limit (1.0Gb) exceeded
```

Here is a version which seems reasonable:

```
split_list_at_nth1(Nth1, Long, Start, End) :-
( nonvar(Nth1) -> must_be(nonneg, Nth1), Once = true
; is_list(Long), (is_list(Start) ; is_list(End)) -> Once = true
; is_list(End), is_list(Start) -> Once = true
; true
),
split_list_at_nth1_(Long, 0, Nth1, Once, Start, End).
split_list_at_nth1_(L, N, N, Once, [], L) :-
(Once == true -> ! ; true).
split_list_at_nth1_([H|T], N, Nth1, Once, [H|Upto], End) :-
N1 is N + 1,
split_list_at_nth1_(T, N1, Nth1, Once, Upto, End).
```

… which produces:

```
?- split_list_at_nth1(N, [a,b,c], [a,b,c|T], End).
N = 3,
T = End, End = [].
```

This produces an unwanted choicepoint, which doesn’t seem worth the expected additional processing to guard against:

```
?- split_list_at_nth1(N, [a,b,c], St, [a,b,c|T]).
N = 0,
St = T, T = [] ;
false.
```

Edit: Removed unnecessary `once`

.