-spec listener(atom() | {atom(), integer() | string()}) -> {ok, pid()} | {error, not_found}.

-spec listeners() -> [{{atom(), integer() | string()}, pid()}].

-spec start_link() ->
                    {ok, Pid :: pid()} |
                    {error, {already_started, Pid :: pid()}} |
                    {error, {shutdown, term()}} |
                    {error, term()} |
                    ignore.