let open_win (wn: win_name) (u: url) (wo: win_opener) (b: b)
: win_ref * b * output_event list =
let (w', b') = build_win wn Blank_url wo None b in
let (wr, b'') = win_new w' b' in
let (b''', oes) = fetch_url u wr b'' in
(wr, b''', UI_win_opened_event :: oes)