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

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

let identity = macro(arg) => arg;