sub busywork { print 'busy' };
my $n = 12;
my $th_sub = sub {
    my ($limit) = @_;
    foreach ( 1 .. $limit ) {
       print '.';
       cede;
    }
};
my $th = Coro->new( $th_sub, $n );
$th->ready;
foreach my $ii ( 1 .. $n ) {
    if ( $ii == int $n * 2/3 ) {
        $Coro::current->prio(-1);
        cede;
        busywork();
    }
    print '-';
    cede;
}