matching?:
	swap 0
	for c in chars:
		if = c "]":
			++
		elseif = c "[":
			if not dup:
				drop
				return false
			--
	not

!. matching? ""
!. matching? "[]"
!. matching? "[][]"
!. matching? "[[][]]"
!. matching? "]["
!. matching? "][]["
!. matching? "[]][[]"