functions in gcd.i - i

    is_prime(x)  


return non-zero if and only if X (which must be a scalar integer)  
is prime.  May return a false positive if X is greater than about  
3e9, since at most 20000 candidate factors are checked.  
The absolute value of X is taken first; zero is not prime, but 1 is.  
Interpreted function, defined at i/gcd.i   line 64