new_contents: def validate(num):
      digits = [int(d) for d in str(num)]
      n = len(digits)
      for i in range(n % 2, n, 2):
          digits[i] *= 2
          if digits[i] > 9:
              digits[i] -= 9
      return sum(digits) % 10 == 0
  
subject: Validate Credit Card Number