BEGIN {
    print pretty_format("3 19 [ + -2 / ] $")
	print pretty_format("[ 1 2 ] [ 3 ]")
	print pretty_format("1")
	print pretty_format("[ 1")
	print pretty_format("1 ]")
}

function pretty_format(str,  b, i, q, ql, r, s, t) {

	ql = split("[ " str " ]", q)

	r = ""
	s = ""
	b = 0
	for(i = 1; i <= ql; i++) {
		t = q[i]
		if (t == "[") {
			r = r s "["
			s = ""
			b++
		}
		else if (t == "]") {
			if (--b < 0) return "error"
			r = r "]"
			s = ", "
		}
		else {
			r = r s "'" t "'"
			s = ", "
		}
	}
	if (b > 0) return "error"

	return r
}