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 }})