let fact = fn n -> 
  begin 
    set (pair 1 n); 
    while snd get > 0 do begin
      let r = fst get in 
      let n = snd get in begin
        if n==10 then break
        else if n==3 then begin
          set (pair r (n-1)); 
          continue
        end 
        else 1;
        set (pair (r*n) (n-1))
      end 
    end; 
    fst get 
  end in 
fact 20