The call gcd m n for two integers m and n returns the (nonnegative)
greatest common divisor of m and n. If m and n are both zero, it
returns zero.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# gcd 10 12;;
val it : int = 2
# gcd 11 27;;
val it : int = 1
# gcd (-33) 76;;
val it : int = 1
# gcd 0 99;;
val it : int = 99
# gcd 0 0;;
val it : int = 0