It wasn't a couple of hours before I ended up using a similar technique as yesterday's post. In this case, it was a multi selection list. Multiple selections were allowed, but some combinations were not allowed. I used the notification property (rather than the verification) and put together a method like this:

clearOtherVersions: aController

	| targetPackage cleanedSelections |

	targetPackage := packagesList list at: aController view targetIndex.

	cleanedSelections := packagesList selections

		reject: [:each | each name = targetPackage name and: [each ~~ targetPackage]].

	packagesList selections: cleanedSelections

Basically, we find what element was just selected, and then reject any other like elements of different versions from the current selection set.