oil language should let you implement xargs -P ?

[andychu]

as a thought experiment.

• this feels like it's missing: wait -n should have a way to retrieve PID
  • you can have a dictionary of PIDs. Or do you need more than that?

[andychu]

Also related to #843

I guess the question is if it should be a fixed thing in C, or if we expose semaphores and let people do it in Oil?

Doing it in Oil would open up the possibility for progress bars and user interfaces ?

Anything else?

[andychu]

This is fairly related to the state machine theme: http://www.oilshell.org/blog/2022/01/notes-themes.html