item.i4.v = menu_bar_update()

	This routine will service a menu bar on the TV screen which
	was created by an earlier call to menu_bar_create.  It returns
	the field number in which the most recent interrupt occurred.
	It will also execute the function (if any) associated with
	that field by menu_bar_create.  It will return a value of zero
	if the most recent interrupt didn't occur in any of the menu
	bar fields.  (Note: This routine shouldn't be called if the call
	to menu_bar_create specified automatic updating.  In that case,
	this routine is called automatically.)

	This function returns status values as follows:

	CBS_NO_SUCH		no menu bar is present
	CBS_WINDOW_OCCLUDED	the menu bar is occluded
	CBS_NOTENB		menu bar is not enabled
	otherwise		return selected item number or zero
				if no item was selected

	This function requires the following include files:

	cbslib_h, acnet_errors_h

	Related functions:

	menu_bar_create, menu_bar_delete, menu_bar_change_entry_c,
	menu_bar_enable_c, menu_bar_disable_c, menu_bar_dncase_entry_c,
	menu_bar_upcase_entry_c, window_has_menu_bar_c,
	window_menu_bar_update_c, menu_bar_min_width_c

	C/C++ usage:

	int	item;

	item = menu_bar_update();