This program could has two names: [[ [[[
[[ ]] is sequential (not required)
[[[ ]]] is parallel
both of them are also used as command quote.

SEE Makefile

if [[ and/or [[[ is used by shell builtin command, use `brackets` as argv[0]

sh script equivalent for simple case:
	#!/bin/sh
	unset pids
	cmds="1 2 3 4"
	for cmd in $cmds; do
	echo ${cmd} &
	pids="$pids $!"
	done
	pwait $pids

	(but what if process terminates and another process with same pid is executed before pwait?
	though pid is not likely to collide.)