Now you've got me confused. If certain things must happen, aren't they neither optional nor alternative?
Optional: You can have chocolate, or not. Also Vanilla, or not.
Altenative: You must choose between chocolate or vanilla, but only one.
Mandatory: You can't order without a cone, Sir. No, Sir, I'm not scooping it into your hand.
Something that must happen is just regular mandatory event, or am I missing something?