R4RS 6.5.5  (gcd integer ...)  ==>  integer
            (lcm integer ...)  ==>  integer

These procedures return the greatest common divisor or least common
multiple of their arguments. The result is always non-negative.

(gcd 32 -36)  ==>  4
(gcd)         ==>  0
(lcm 32 -36)  ==>  288
(lcm)         ==>  1
