1 def simp(r: Rexp): (Rexp, Val => Val) = r match {

     2   case SEQ(r1, r2) => {

     3     val (r1s, f1s) = simp(r1)

     4     val (r2s, f2s) = simp(r2)

     5     (r1s, r2s) match {

     6       case (NULL, _) => (NULL, F_ERROR)

     7       case (_, NULL) => (NULL, F_ERROR)

     8       case (EMPTY, _) => (r2s, F_SEQ_Void1(f1s, f2s))

     9       case (_, EMPTY) => (r1s, F_SEQ_Void2(f1s, f2s))

    10       case _ => (SEQ(r1s,r2s), F_SEQ(f1s, f2s))

    11     }

    12   }

    13   ...

    14 

    15 def F_SEQ_Void1(f1: Val => Val, f2: Val => Val) = 

    16   (v:Val) => Sequ(f1(Void), f2(v))

    17 def F_SEQ_Void2(f1: Val => Val, f2: Val => Val) = 

    18   (v:Val) => Sequ(f1(v), f2(Void))

    19 def F_SEQ(f1: Val => Val, f2: Val => Val) = 

    20   (v:Val) => v match {

    21     case Sequ(v1, v2) => Sequ(f1(v1), f2(v2)) }