... inChapter 2, we feel free to use them in illustrative examples.4This set could be called “the relation of being married”. 16 Chapter 1. Some Set Theoretical NotionsSince relations are sets, the equality ... are used instead. Note that, if Aand B have some elements in common, these elements need not be mentionedtwice when forming the union A ∪B. The difference A − B is also called thecomplement of ... set consisting of some elements a, b, c, , (recall that these elements are notregarded as elements of A;thusA consists of a single element p,whereasp mayhave many elements; A and p then are...