infixr 4 ++
infixr 6 **;
functor COMPILE( structure Expn:ExpnSig 
                       and Machine:MachineSig
               ):CompileSig =
struct
      open Expn Machine
      fun codeacc (Id s,   rest) = PushVal s :: rest
        | codeacc (Lit n,  rest) = PushLit n :: rest
        | codeacc (a ++ b, rest) = codeacc(a, codeacc(b, Add :: rest))
        | codeacc (a ** b, rest) = codeacc(a, codeacc(b, Mul :: rest))

   fun code expn = codeacc(expn, []) 
end;