xo-expression: + Expression::nested_layer()

include/xo/expression/Environment.hpp

@@ -6,10 +6,12 @@
 #pragma once
 
 #include "xo/refcnt/Refcounted.hpp"
-#include "Variable.hpp"
+#include "binding_path.hpp"
 
 namespace xo {
     namespace ast {
+        class Expression;
+
         class Environment : public ref::Refcount {
         public:
             /** true if this is toplevel (global) environment.