a) The minus operator on sets is usually defined as: A - B = {x | x ? A and x ? B}. Prove that the class of sets accepted by finite automata is closed under minus. b) Show that the regular sets are not closed under infinite union by producing an infinite family of regular sets whose union is not regular. c) Design an algorithm to determine whether a finite automaton accepts an infinite set. Prove that your algorithm is correct. d) Exhibit an algorithm that detects whether one finite automaton accepts a subset of the set accepted by another machine. Show that this procedure works.