perl6 -e 'await do for 1 .. 20 { start { say "$_.."; 3.sleep; say "$_..done" } }'
1..
2..
3..
4..
5..
6..
7..
8..
9..
10..
11..
12..
13..
14..
15..
16..
17..
Unhandled exception: Cannot call method 'count' on a null object
   at <unknown>:1  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm:throw:4294967295)
 from src/gen/m-CORE.setting:18728  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm:result:113)
 from src/gen/m-CORE.setting:20089  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm:await:21)
 from src/gen/m-CORE.setting:20088  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm:await:25)
 from src/gen/m-CORE.setting:7880  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm::574)
 from src/gen/m-CORE.setting:7803  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm:reify:53)
 from src/gen/m-CORE.setting:7716  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm::294)
 from src/gen/m-CORE.setting:7688  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm:reify:53)
 from src/gen/m-CORE.setting:8205  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm:gimme:193)
 from src/gen/m-CORE.setting:8664  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/CORE.setting.moarvm:sink:4294967295)
 from src/gen/m-main.nqp:41  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/perl6.moarvm:MAIN:34)
 from src/gen/m-main.nqp:35  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/perl6.moarvm:<mainline>:197)
 from <unknown>:1  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/perl6.moarvm:<main>:8)
 from <unknown>:1  (/Users/ec/.rakudobrew/moar-HEAD/install/languages/perl6/runtime/perl6.moarvm:<entry>:9)