item.i4.v = window_menu_bar_update_c(window_id.i2.v)

	This routine will service a menu bar on the TV screen which
	was created by an earlier call to 'window_menu_bar_create(_c)'.
	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 'window_menu_bar_create(_c)'.  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 'window_menu_bar_create(_c)' specified automatic
	updating.  In that case, this routine is called automatically.)

	window_id	ID of desired window

	This function returns status values as follows:

	CBS_NO_SUCH		no menu bar is present
	CBS_WINDOW_OCCLUDED	menu bar window 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_update, window_menu_bar_create_c, window_menu_bar_delete_c,
	menu_bar_enable_c, menu_bar_disable_c, menu_bar_upcase_entry_c,
	menu_bar_dncase_entry_c, menu_bar_change_entry_c,

	C/C++ usage:

	short	window_id = WMNGR_BACKGROUND;
	int	status;

	status = window_menu_bar_update_c(window_id);