Actually using [ZLang](https://github.com/pfultz2/ZLang), you could define them like this:

    #define IOD_DEFINE_SYMBOL(name) iod_define_symbol(name, _##name)
    #define IOD_S(name) s::_##name

    #define ZLANG_iod_define (IOD_DEFINE_SYMBOL)
    #define ZLANG_iod_s (IOD_S)

And then compiling with `-DZLANG_NS=iod` you can call it like this:

    // Define symbols
    $(define hello)
    $(define message)

    auto hello_api = make_api(

        // The hello world procedure.
        $(s hello) = [] () { return D($(s message) = "Hello world."); }

    );

Also, ZLang can be supported without a need for a dependency on ZLang.