Greatest Common Divisor

Using Euclid's algorithm, implement the function

val gcd : int -> int -> int

so that gcd m n is the greatest common divisor of m and n.

Examples

let _ = assert (gcd 12 8 = 4)
let _ = assert (gcd 17 5 = 1)
let _ = assert (gcd 0 9 = 9)
let _ = assert (gcd 48 18 = 6)
let _ = assert (gcd 100 75 = 25)