When an entry on my menu is selected, I have
a submenu consisting of 3 entries. The menu
works fine but I'd like to add the following
to the entries:

When user clicks to select any of the 3 entries,
I would like to show a check mark (a tick) on the
left of the entry to indicate that it is selected.
When user changes the selection, I'd like the
new one marked and the old one un-marked. How
can I achieve that?

Jang