fun safe x y ((x1,y1)::xs) = 
                x <> x1          (* not same column   *)
        andalso y <> y1          (* not same row      *)
        andalso y - x <> y1 - x1 (* not on /-diagonal *)
        andalso y + x <> y1 + x1 (* not on \-diagonal *)
        andalso safe x y xs 
  | safe _ _  _            = true

exception Conflict

fun queens4 n =
    let fun queens x y posl =
            if x > n then posl                 (*   1 *)
            else if y > n then raise Conflict  (* 2.b *)
            else (if safe x y posl              (* 2.a *) 
                  then queens (x+1) 1 ((x,y) :: posl)
                  else raise Conflict)
                 handle Conflict => queens x (y+1) posl
    in  queens 1 1 []
    end

val doit = let val sol =
                   queens4 (valOf(Int.fromString(hd(CommandLine.arguments()))))
           in  app (fn(x,y) => app print ["(",Int.toString x,", ",
                                          Int.toString y,")\n"]) sol
           end