2 gcd - greatest common divisor of a set of rational numbers
8 x1, x2, ... rational number
10 return rational number
13 If at least one xi is nonzero, gcd(x1, x2, ...) is the
14 greatest positive number g for which each xi is a multiple of g.
15 If all xi are zero, the gcd is zero.
18 > print gcd(12, -24, 30), gcd(9/10, 11/5, 4/25), gcd(0,0,0,0,0)
22 The number of arguments may not to exceed 100.
25 NUMBER *qgcd(NUMBER *x1, NUMBER *x2)