A+ Promise

role APlus {
    method andthen(&block) {
        nextwith -> Promise $p {
            my \res = block $p.result;
            do if res ~~ Awaitable {
                await res
            } else {
                res
            }
        }
    }
}
sub a-start(&block) { (start block) does APlus }

say await (a-start { 1 }).andthen(-> Int $i { a-start { $i + 1 }})