If you turn off the menu option O_ONEVALUE (e.g., with
set_menu_opts or menu_opts_off; see menu_opts(3X)), the menu
becomes multi-valued; that is, more than one item may simultaneously be
selected.
In a multi_valued menu, you can used set_item_value to select the
given menu item (second argument TRUE) or deselect it (second argument
FALSE).
RETURN VALUE
The function set_item_value returns one of the following:
