Set value of 'Choice - Multiple' control through rules

I've come across a use case in which it would be helpful if checking a box in one choice field would uncheck a box in a previous choice field. In other words, clicking the box in the second field invalidates the checked box in the first.

Also, I haven't come across such a need myself, but I could also see the need for the ability to hide or "gray out" certain options in a multi-choice field based on one of the other options being selected. For example, if my options were 1,2,3,4,5 and the user selects 3, the 4 and 5 options are then removed because they no longer apply or whatever.

In other words, rules for choice fields ...