diff --git a/schala-lang/language/src/ast/visitor.rs b/schala-lang/language/src/ast/visitor.rs
index 6e74596..13dd73c 100644
--- a/schala-lang/language/src/ast/visitor.rs
+++ b/schala-lang/language/src/ast/visitor.rs
@@ -46,6 +46,8 @@ pub enum VisitorOutput<T, E> {
   Err(E)
 }
 
+//TODO - cf. the example about "Tree Construction Visitors", every enum type needs its own Visitor
+//trait
 pub trait ExpressionVisitor {
   type Output;
   fn type_anno(&mut self, _anno: &TypeIdentifier) -> Self::Output;