Many entities are "linked" together in some way. An cat might have an owner, or a snowball might have a thrower, or a zombie might have an aggro target. I propose a way to test and modify these links with commands.
/link <entity1> <linkType> set <entity2>
/link <entity1> <linkType> test [<entity2>]
/link <entity> <linkType> remove
/execute linked <linkType> -> execute