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