If click in the edit line and type =index( the tool tip shows that INDEX can have 5 arguments with the last one not named (...). This is due to:

595		str = g_string_new (gnm_func_get_name (fd));
(gdb) 
596		g_string_append_c (str, '(');
(gdb) p str
$1 = (GString *) 0x8507820
(gdb) p str->str
$2 = (gchar *) 0x84336b8 "index"
(gdb) n
598		function_def_count_args (fd, &min, &max);
(gdb) 
599		for (i = 0; i < max; i++) {
(gdb) p min
$3 = 0
(gdb) p max
$4 = 2147483647

So apparently index can have an arbitrary number of arguments....