Concurrency

sub fib-promise-v2a($n, $depth) { if $depth > 0 { fib-promise-v2a($n - 1, $depth - 1), fib-promise-v2a($n - 2, $depth - 1); } else { fib-promise-v2b($n - 1), fib-promise-v2b($n - 2); } }