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))+ −
}+ −