head, *tail = chars
if head.isdigit():
    if not pos % 2:
        return Luhn.luhny_bin(pos + 1, sum + int(head), tail)
    else:
        return Luhn.luhny_bin(pos + 1, sum + Luhn.luhny_tune(int(head)), tail)
if head == " ":
    return Luhn.luhny_bin(pos, sum, tail)
