Checks if credit card number fits the discover card format.

def is_discover(n):
    """Checks if credit card number fits the discover card format."""
    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