Concurrency

... when 1 { return start { fib-promise($n - 1, 0) }, start { fib-promise($n - 2, 0) }; } when * > 0 { return fib-promise($n - 1, $depth - 1), fib-promise($n - 2, $depth - 1); } } return fib-promise($n - 1, 0) + fib-promise($n - 2, 0); }