title: Finding difference between two very large lists
body: <p>I have two very large lists, one is 331991 elements long, lets call that one a, and the other is 99171 elements long, call it b. I want to compare a to b and then return a list of elements in a that are not in b. This also needs to be efficient as possible and in the order that they appear, that is probably a given but I thought I may as well throw it in there.</p>