type html_tag =
  [ `BR
  | `L of html_tag list
  | `T of string
  | `DIV of int * string option * html_tag  (* indentation, color, tag *)
  ]
type html_msg = [ `Error of html_tag | `Msg of html_tag ]

val string_of_html_msg: html_msg -> string
val html_of_html_msg: html_msg -> string

type logger_fun = ?append_NL:bool -> html_msg -> unit

val register_log_callback: logger_fun -> unit

val log: logger_fun