Function: gcd
Section: number_theoretical
C-Name: gcd0
Prototype: GDGD0,L,
Help: gcd(x,{y}): greatest common divisor of x and y.
Description: 
 (small, small, ?0):small	cgcd($1, $2)
 (int, int, ?0):int	        gcdii($1, $2)
 (gen, gen, ?0):gen	        ggcd($1, $2)
 (gen, gen, 1):gen	        modulargcd($1, $2)
 (gen, gen, 2):gen	        srgcd($1, $2)
 (gen, ?gen, ?small):gen	gcd0($1, $2, $3)

