datatype instruction = 
   PUSH of result | TIMES_OP | DIV_OP | PLUS_OP | SUB_OP ;

fun genCode Empty = ([], [])
 |  genCode (EXP(termTree, exp1Tree)) =
      let val (tFirst, tRest) = genCode termTree in
       let val (eFirst, eRest) = genCode exp1Tree in 
        ((tFirst @ tRest), (eFirst @ eRest))
       end
      end
 |  genCode (EXP1(OP(PLUS), expTree)) = 
      let val (first, Rest) = genCode expTree in
        ((first @ [PLUS_OP]), Rest)
      end
 |  genCode (EXP1(OP(SUB), expTree)) =
      let val (first, Rest) = genCode expTree in
        ((first @ [SUB_OP]), Rest)
      end
 |  genCode (TERM(factorTree, term1Tree)) = 
      (* note that the factor could in fact be 
         an expression in brackets. In this case 
         we need to collect first, too
      *)
      let val (fFirst,fRest) = genCode factorTree in
       let val (tFirst, tRest) = genCode term1Tree in
         ((fFirst @ fRest), (tFirst @ tRest))
       end
      end
 |  genCode (TERM1(OP(DIV), termTree)) = 
      let val (first, rest) = genCode termTree in
        ((first @  [DIV_OP]), rest)
      end
 |  genCode (TERM1(OP(TIMES), termTree)) = 
      let val (first, rest) = genCode termTree in
        ((first @ [TIMES_OP]), rest)
      end
 |  genCode (FACTOR(X)) = ([], [PUSH(X)]);

fun translate () = 
  let val (expFirst, expRest) = genCode(parse()) in
    (expFirst @ expRest)
  end

fun translate1 x = 
  let val (expFirst, expRest) = genCode(x) in
    (expFirst @ expRest)
  end;