code: def is_discover(n):
      
      n, length = str(n), len(str(n))
  
      if length == 16:
          if n[0] == '6':
              if ''.join(n[1:4]) == '011' or n[1] == '5':
                  return True
              elif n[1] == '4' and n[2] in strings_between(4, 10):
                  return True
              elif ''.join(n[1:6]) in strings_between(22126, 22926):
                  return True
      return False
docs: Checks if credit card number fits the discover card format.