diff --git a/includes/tree.h b/includes/tree.h
index 2cfe2357..1f967f2b 100644
--- a/includes/tree.h
+++ b/includes/tree.h
@@ -77,6 +77,7 @@ enum expr_op {
 	expr_const_int,
 	expr_exists,
 	expr_encapsulate,
+	expr_known,
 };
 
 struct expression {