let win_from_user_window (User_window(u, n): user_window) (b: b)
: win_ref option =
let has_url (_, w) =
(page_assoc_valid w.win_page b).page_location = u
in
let windows' = List.filter has_url b.browser_windows in
begin if List.length windows' <= n then
None
else
Some (fst (List.nth b.browser_windows n))
end