let maybe = module{
    val = NULL,
} => {
    let do = macro(op) => {
        result = select (mod.val != NULL), NULL, {
            true = op(mod.val).result,
        }
    };
};

let if = module{
    test = false,
} => {
    let do = macro(op, arg) => {
        result = select mod.test, arg, {
            true = op(arg).result,
        },
    };
};

let identity = macro(arg) => {
    result = arg,
};