diff --git a/app/Compiler.hs b/app/Compiler.hs
index c510535..849afdb 100644
--- a/app/Compiler.hs
+++ b/app/Compiler.hs
@@ -99,4 +99,5 @@ goal2struct x = x
 
 struct2goal :: PrlgInt -> PrlgInt
 struct2goal (AtomI s) = CallI s []
-struct2goal x = x
+struct2goal call@(CallI _ _) = call
+struct2goal _ = error "TODO."