@jreidinger One solution that would keep things simple from `Command`'s POV and still avoid hidden global variable would be to pass some `ParallelRunner` to `Command` with the number of threads already set. `Command#apply` would then just call `ParallerRunner#in_parallel`. This solution is really just DI used correctly. But it may be an overkill in this case.

@lslezak As the opinions differ, I'd say it's up to you. I don't think it matters much in practive which way we'll choose (as `yk` is pretty small).