Ran into something pretty slick today with Ruby arrays. The & operator and the - operator.
So if you do something along the lines of [1,2,3,4,5] & [4,5,6,7] you'll get the intersection of the two arrays [4,5].
And if you do something like [1,2,3,4,5] - [3,4,5] you'll get the difference of the two arrays [1,2].