def gcd(a, b):
    if not isinstance(a, int):
        raise TypeError('a must be integral')
    if a < 0:
        raise ValueError('a must be nonnegative')
    if not isinstance(b, int):
        raise TypeError('b must be integral')
    if b < 0:
        raise ValueError('b must be nonnegative')

    # ...time for the real work...