Dead end with macros

src/utils/visitor.rs

@@ -0,0 +1,18 @@
+pub trait Visit<T> {
+  type Return;
+  fn visit(&self, target: T) -> Return; 
+}
+
+pub trait ImpureVisit<T> {
+  type Shard;
+  type Return;
+  fn impure_visit(&self, target: T) -> (Shard, Return);
+  fn merge(&mut self, s: Shard);
+}
+
+pub struct OverlayVisitor<VBase, VOver>(VBase, VOver);
+
+impl<VBase, VOver, T, R> Visitor<T> for OverlayVisitor<VBase, VOver>
+where VBase: Visitor<T, Return = Option<R>>, VOver: Visitor<T, Return = Option<R>> {
+  
+}