diff --git a/examples/exact-cover.lp b/examples/exact-cover.lp
index 396f69b..cd4990d 100644
--- a/examples/exact-cover.lp
+++ b/examples/exact-cover.lp
@@ -2,4 +2,4 @@
 
 :- I != J, in(I), in(J), s(X, I), s(X, J).
 covered(X) :- in(I), s(X, I).
-:- s(X, Y), not covered(X).
+:- s(X, I), not covered(X).