let solutions n =

  let show board =
    let pr v =
      for i = 1 to n do
        print_string (if i=v then " q" else " _");
      done;
      print_newline() in
    List.iter pr board;
    print_newline() in

  let rec safe i j k = function
    | [] -> true
    | h::t -> h<>i && h<>j && h<>k && safe i (j+1) (k-1) t in

  let rec loop col p =
    for i = 1 to n
    do
      if safe i (i+1) (i-1) p then
        let p' = i::p in
        if col = n then show p'
        else loop (col+1) p'
    done in

  loop 1 [] in

let n =
  if Array.length Sys.argv > 1
  then int_of_string Sys.argv.(1)
  else 8 in

solutions n