const Chain(comptime T: type, comptime U: type) = struct {
    t T,
    u U,
    onU bool,
    fn init(t: T, u: U) @This() {
        return @This() {
            .t = t,
            .u = u,
            .onU = false,
        };
    }
    fn next(self: *@This()) @typeOf(T.next).ReturnType {
        return if (self.onU) self.u.next() else self.t.next();
    }
};