xo-expression: initial commit (constant + primitive)

example/ex1/ex1.cpp

@@ -0,0 +1,14 @@
+/** @file ex1.cpp **/
+
+#include "xo/expression/Constant.hpp"
+#include <iostream>
+
+int
+main() {
+    using xo::ast::make_constant;
+
+    auto expr = make_constant(7);
+}
+
+
+/** end ex1.cpp **/