I don't know which one it is. I've never used the STL before, and I'm just getting started, and finding that using it doesn't appear to be horribly difficult. I just need clarification on one thing... which one of the libraries is like a linked list, but only adds a new element if it doesn't exist anywhere else in the list? Like, if I tried to added "1,2,3,4,5,6" to the list, and then tried to add 5 again, it wouldn't do anything. Hmmm... I think there is one. Am I wrong?