diff --git a/day3.m b/day3.m
index c8eea59..77cd651 100644
--- a/day3.m
+++ b/day3.m
@@ -35,8 +35,11 @@ prio(C) = I :-
 :- type triple(T) ---> t(T, T, T).
 
 :- pred threes(list(T)::in, list(triple(T))::out) is semidet.
-threes([],               []).
-threes([X, Y, Z | Rest], [t(X, Y, Z) | Groups]) :- threes(Rest, Groups).
+threes(In, Out) :- threes(Out, In, []).
+
+:- pred threes(list(triple(T))::out, list(T)::in, list(T)::out) is semidet.
+threes([])                  --> =([]).
+threes([t(X, Y, Z) | Rest]) --> [X, Y, Z], threes(Rest).
 
 
 :- pred go1(string::in, int::out) is cc_multi.
@@ -63,4 +66,4 @@ run(two, Lines, univ(sum(Prios))) :-
   if threes(Lines, Groups) then
     map(go2, Groups, Prios)
   else
-    die("not a multiple of three lines").
+    die("%d lines given (not a multiple of 3)", [i(length(Lines))]).