"Well, how is your processor?" "It's OK. I sat in liquid nitrogen for an hour, so now I'm as good as new!" "Great. Then let's continue." "Set collections." Mathematically speaking, a set is a group of unique elements. Thus, in programming, a Set is a collection of unique elements, i.e. a collection that doesn't let you store identical elements. "I don't know if Ellie showed you Set's inheritance hierarchy. If not, here it is ..."