functor RPerm(ArraySig) =
struct
fun rperm n = 
let val a = tabulate (n, fn x => x)
    fun swap(i,j) = let val t = a sub i 
                    in( update(a,i,a sub j); 
                        update(a,j,t)) 
                    end
    fun permute 0 = ()
      | permute s = (swap(rand s, s-1); 
                     permute (s-1))
in
    (permute n; a)
end