sub fib {
    my $a = 0;
    my $b = 1;
    
    gather {
        loop {
            take $a;
            my $c = $a + $b;
            $a = $b;
            $b = $c;
        }
    }
}

# (0 1 1 2 3 5 8 13 21 34 55)
fib[10] == 55 or die "Wrong value";