my Supply $out-supply = supply {
  my Proc::Async $proc-async .= new(</bin/ls -l>);
  $proc-async.stdout.lines.tap({ say 'forwarding ' ~ $_; emit($_) });
  my Proc $done-proc = await $proc-async.start;
  say 'process exited';
  if $done-proc.exitcode != 0 {
    say 'bad exit code';
    emit(
      Failure.new(
	"Process exited with non-zero exit code: {$done-proc.exitcode}"
      )
    );
  }
  say 'done';
}

say 'i have control';
say $out-supply.list.Array;