Find solutions to the n-queens problem

NQueens.tq

let safe (x1, y1) (x2, y2) =
  x1 <> x2 && y1 <> y2 && x2 - x1 <> y2 - y1 && x1 - y2 <> x2 - y1

let rec next n s =
  Stack.pop s @
  [ None -> None
  | Some((qs, ps), s) ->
      Stack.pop ps @
      [ None -> if Stack.length qs = n then Some(qs, s) else next n s
      | Some(q, ps) ->
          Stack.push (qs, ps) s
          @ Stack.push (Stack.push q qs, Stack.filter (safe q) ps)
          @ next n ] ]

let ps n =
  for 1 1 n (Stack.empty()) [ps, i ->
    for 1 1 n ps [ps, j ->
      Stack.push (i, j) ps]]

let tableOf (n, qs) =
  let qs = Set.ofStack qs in
  Array.init n [i ->
    Array.init n [j ->
      Html.of(if Set.contains (i+1, j+1) qs then "♕" else "")
      @ Html.tag "span" {"style", "font-size: 24px"}
      @ Html.tag "td" {"style", "border: 1px solid gray; width: 28px; text-align: center"} ]
    @ Html.concat
    @ Html.tr ]
  @ Html.concat
  @ Html.tag "table" {"style", "border: 2px solid black; margin: auto;"}
  @ yield

let solutions n = Seq.unfold (next n) (stack{Stack.empty(), ps n})

let f n =
  solutions n
  @ Seq.first
  @ Option.iter [qs -> tableOf(n, qs)]