primes prints the sequence of prime numbers:
2, 3, 5, 7, 11, and so on,
one per line.
low is supplied,
primes does not print any primes smaller than
high is supplied,
primes does not print any primes larger than
high is not supplied,
primes stops at 1000000000;
the last prime is 999999937.
primes can handle
high as large as 1000000000000000.