def rev(r: Rexp) : Rexp = r match {+− case ZERO => ZERO+− case ONE => ONE+− case CHAR(c) => CHAR(c)+− case ALT(r1, r2) => ALT(rev(r1), rev(r2))+− case SEQ(r1, r2) => SEQ(rev(r2), rev(r1))+− case STAR(r) => STAR(rev(r))+− }+−