| View of a binary operation between two sets. Handles union, intersection, diff and sym_diff Instead of copying the sets, it merely maintains pointers to the two sets. This view is read-only and *cannot* be used to modify the original sets. Note that it is *not* a value interface. In fact, if the original sets change, this view will automatically change. In some cases this is exactly the behavior you want; in other cases it can be a source of nasty problems. Use carefully.