Concurrency
Splitting the first concurrent version into four functions.
sub fib-promise-v2($n, $depth = 5) {
my @promises = (
fib-promise-v2a($n - 2, $depth - 1),
fib-promise-v2a($n - 1, $depth - 1),
).flat;
await Promise.allof(@promises);
return [+] @promises>>.result;
}