Let \(\mathscr{D}\) be a Dynkin system. Show that for all \(A, B \in \mathscr{D}\) with \(A \subset B\) the difference \(B \backslash A \in \mathscr{D}\). [ use \(R \backslash Q=\left((R \cap Q) \cup R^{c}ight)^{c}\), where \(R, Q \subset X\).]