let one_of = module {
    val=NULL,
    types=[],
} => {
    let reducer = macro(acc, t) => acc{
        ok = acc.ok || (acc.val is t),
    };
    let any = macro(val, types) => reduce reducer {ok=false, val=val}, types;

    let result = any(mod.val, mod.types).ok;
};

let must = macro(m, msg) => select m.result, fail msg, {
    true = m.result,
};