diff --git a/ast.go b/ast.go
index 43969d5..cde43ec 100644
--- a/ast.go
+++ b/ast.go
@@ -12,6 +12,7 @@ import (
 type Expr interface {
 	Eval(ctx ExprContext) (result any, err error)
 	eval(ctx ExprContext, preset bool) (result any, err error)
+	String() string
 }
 
 //-------- ast