xo-expression2: + Binding etc

include/xo/expression2/Binding.hpp

@@ -0,0 +1,46 @@
+/** @file Binding.hpp
+ *
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#pragma once
+
+#include <cstdint>
+
+namespace xo {
+    namespace scm {
+        class Binding {
+        public:
+            static constexpr int32_t s_link_sentinel = -2;
+            static constexpr int32_t s_link_global = -1;
+
+        public:
+            Binding(int32_t i_link, int32_t j_slot)
+                : i_link_{i_link}, j_slot_{j_slot} {}
+
+            /** global bindings are located by symbol name **/
+            static Binding global() { return Binding(s_link_global, 0); }
+            static Binding local(int32_t j_slot) { return Binding(0, j_slot); }
+
+            bool is_global() const { return i_link_ == s_link_global; }
+
+            int32_t i_link() const noexcept { return i_link_; }
+            int32_t j_slot() const noexcept { return j_slot_; }
+
+        private:
+            /**
+             *  >= 0: number of parent links to traverse
+             *        to a fixed-size frame
+             *    -1: resolve globally
+             **/
+            int32_t i_link_ = s_link_sentinel;
+            /** if @ref i_link_ >= 0, frame offset
+             *  (in 'variables' not bytes).
+             *  ignored if @ref i_link_ is global
+             **/
+            int32_t j_slot_ = -1;
+        };
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* Binding.hpp */