Concurrency

sub fib-promise-v2b($n) { start { fib-promise-v2c($n - 1) }, start { fib-promise-v2c($n - 2) }; } sub fib-promise-v2c($n) { $n > 1 ?? fib-promise-v2c($n - 1) + fib-promise-v2c($n - 2) !! $n; }