Add 'xo-expression/' from commit '5ac3c03a0c'

git-subtree-dir: xo-expression
git-subtree-mainline: d0f5ccc1ce
git-subtree-split: 5ac3c03a0c

xo-expression/include/xo/expression/binding_path.hpp

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+/* file binding_path.hpp
+ *
+ * author: Roland Conybeare, Jul 2024
+ */
+
+#pragma once
+
+namespace xo {
+    namespace ast {
+        /** @class path
+         *
+         *  @brief path from the *use* of a variable to the environment
+         *  providing its location.
+         **/
+        struct binding_path {
+            /** @of parent links to traverse.  -1 if global. -2 if sentinel **/
+            int i_link_ = -2;
+            /** for variables bound in some local environment:
+             *  slot# within that environment.
+             *
+             *  Ignored if @ref i_link_ is -1
+             **/
+            int j_slot_ = 0;
+        }; /*binding_path*/
+    } /*namespace ast*/
+} /*namespace xo*/
+
+
+/* end binding_path.hpp */