Let me repeat if I understand it right: * The function will return True for all primes. * The function will return False if the number is detected as a compos

Let me repeat if I understand it right: * The function will return True for all primes. * The function will return False if the number is detected as a composite by some tests. * The function can return True or False for all other numbers. * For numbers<=23 they implemented a shortcut using a lookup table which is implemented as bits in a number. * The bit for number 19 is wrong. It returns false for a prime number which violates "return True for all prime numbers". This is indeed quite shoddy programming for such an essential and easily testable piece of software.