let string_of_output output =
match output with
| UI_win_opened_event -> "UI_win_opened_event"
| UI_win_closed_event(user_window) ->
"UI_win_closed_event"
| UI_page_loaded_event(user_winndow, url, rendered_doc) ->
"UI_page_loaded_event with " ^ (string_of_url url)
| UI_page_updated_event(user_window, rendered_doc) ->
"UI_page_updated_event"
| UI_alert(text)-> "UI_alert"
| UI_error(text) -> "UI_error"
| Network_send_event (domain, req) ->
"Network_send_event with " ^ (string_of_domain domain)