How do I limit the number of active threads (queuing spawn calls)

[Andrej Mitrovic]

Edit: It looks like I did almost the same as Jonathan advised.

I'm looking forward to std.parallelism though. I'm thinking I'd
probably use some kind of parallel foreach loop that iterates over 4
files at once, and letting it do its work by spawning 4 threads. Or
something like that. We'll see.