Function: isprime
Section: number_theoretical
C-Name: gisprime
Prototype: GD0,L,
Help: isprime(x,{flag=0}): true(1) if x is a (proven) prime number, false(0)
 if not. If flag is 0 or omitted, use a combination of algorithms. If flag is
 1, the primality is certified by the Pocklington-Lehmer Test. If flag is 2,
 the primality is certified using the APRCL test.
Description: 
 (int, ?0):bool	isprime($1)
 (int, 1):bool	plisprime($1, 0)
 (int, 2):gen	plisprime($1, 1)
 (gen, ?small):gen	gisprime($1, $2)

