interpreter_tree first iteration

include/visitor.hpp

@@ -0,0 +1,16 @@
+#pragma once
+
+// for clangd
+#include "interpreter_tree.hpp"
+
+namespace interpreter {
+
+class Visitor {
+public:
+  virtual void Visit(Node* node) = 0; // ??
+  //  visit(SomethingNode node)
+  // ...
+private:
+};
+
+} // namespace interpreter