1 def rev(r: Rexp) : Rexp = r match {

     2   case ZERO => ZERO

     3   case ONE => ONE

     4   case CHAR(c) => CHAR(c)

     5   case ALT(r1, r2) => ALT(rev(r1), rev(r2))

     6   case SEQ(r1, r2) => SEQ(rev(r2), rev(r1))

     7   case STAR(r) => STAR(rev(r))

     8 }