my Proc::Async $proc-async .= new(</bin/false>);
my Supply $supply-lines = $proc-async.stdout.lines;
my Promise $promise-done = $proc-async.start;

my Supply $out-supply = supply {
    whenever $supply-lines -> Str $line {
      say 'forwarding ' ~ $line;
      emit $line;
    }

    whenever $promise-done -> Proc $done-proc {
      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;