utils.elpi

% ---- Utils lib ----

length [] 0.
length [_|TL] L :- length TL L', L is L' + 1.

nth_element 1 [(X, _)|_] X.
nth_element N [_|TL] X :- 
    M is N - 1,
    nth_element M TL X.

append [] L L.
append [X|XS] L [X|YS] :- append XS L YS.
append X L [X|L].

append_no_dups [] L L.
append_no_dups [X|XS] L [X|YS] :- not (mem X L), append_no_dups XS L YS.
append_no_dups [X|XS] L YS :- append_no_dups XS L YS.

mem _ [] :- !, fail.
mem E [E|_].
mem E [_|L] :- mem E L.

reverse [] [].
reverse [X|XS] L :-
    reverse XS L',
    append L' [X] L.