TYPE

  Even = flat(snum)
  State  = Var -> Even

PROBLEM Parity

  direction  : forward
  carrier    : State
  init       : bot
  init_start : [-> top]
  combine    : lub


TRANSFER

// in assignments calculate the new value of the variable and add it to the state
  ASSIGN(variable, expression) = 
      @\[variable -> evalAExp(expression, @)]

// in procedur calls pass the value of the actual argument to the formal parameter
  CALL(_, param, exp), call_edge = 
      @\[param -> evalAExp(exp, @)]

  CALL(_, _, _), local_edge = bot


// at the end of procedures reset the formal parameter
  END(_, param) =
      @\[param -> top]

SUPPORT

  evalAExp :: Expression * State -> Even
  evalAExp(expression, state) =
    case expType(expression) of
      "ARITH_BINARY" => case expOp(expression) of
        "+" => let valLeft  <= evalAExp(expSubLeft(expression),  state),
                   valRight <= evalAExp(expSubRight(expression), state) in
                 if ((valLeft=0 && valRight=1)
                               ||(valLeft=1 && valRight=0)) 
                 then lift(1)
                 else
                 if ((valLeft=0 && valRight=0)
                               ||(valLeft=1 && valRight=1)) 
                 then lift(0)
                 else top endif endif;
        "-" => let valLeft  <= evalAExp(expSubLeft(expression),  state),
                   valRight <= evalAExp(expSubRight(expression), state) in
                 if ((valLeft=0 && valRight=1)
                               ||(valLeft=1 && valRight=0)) 
                 then lift(1)
                 else
                 if ((valLeft=0 && valRight=0)
                               ||(valLeft=1 && valRight=1)) 
                 then lift(0)
                 else top endif endif;
        "*" => let valLeft  <= evalAExp(expSubLeft(expression),  state),
                   valRight <= evalAExp(expSubRight(expression), state) in
                 if (valLeft=0 || valRight=0)
                 then lift(0)
                 else
                 if (valLeft=1 && valRight=1) 
                 then lift(1)
                 else top endif endif;
        "/" => top;
        endcase;
      "ARITH_UNARY" =>
	case expOp(expression) of
	  "-" => evalAExp(expSub(expression), state);
        endcase;
      "VAR"   => state ( expVar(expression) );
      "CONST" => lift(expVal(expression)%2);
      _       => error("Runtime Error: evalAExp applied to nonarithmetic Expression");
    endcase