All items in the origin set are moved to the destination set
Only items with a value greater than 9 are moved to the destination set.
Additionally destination items with value greater than 19 are removed from the set (but NOT removed from memory).
If you want to remove them from memory then use destination.remove
Suppose we have two collections of items and we want to make sure that one collection has all the small value items and the other has the high value items.
I.e. the process of moving items around will change the criteria for subsequent items.
Move an item from one set to another if it increases the average value of both sets.