if x.l != nil {
		x.s = strings.Join(reverse(x.appendReverse(nil)), "")
		x.l = nil
		x.r = nil
	}
	s := x.s
	x.mu.Unlock()

	return s
}

// reverse reverses x in place and returns it.
func reverse(x []string) []string {
	n := len(x)
	for i := 0; i+i < n; i++ {
		x[i], x[n-1-i] = x[n-1-i], x[i]
	}
	return x
}

// appendReverse appends to list all of x's subpieces, but in reverse,