Sorry for coming up with that now, but I just realized that Scala's `diff` has `EXCEPT ALL` semantics (not removing duplicates), not EXCEPT semantics (removing duplicates). http://en.wikipedia.org/wiki/Set_operations_%28SQL%29#EXCEPT_operator

So contrary to what I said before we should not call this `diff`. Maybe `except` is the most suitable name after all. @szeiger WDYT?