anchor.diff:      ;
   
   additiveExpression returns [Term term]
  -    : me1=multiplicativeExpression
  +    : me1=scaleExpression
         { $term = $me1.term; }
         (
           additiveOperator 
           { $additiveOperator.operator.setLeftTerm($term);
             $term = $additiveOperator.operator; }
  -        me2=multiplicativeExpression
  +        me2=scaleExpression
           { $additiveOperator.operator.setRightTerm($me2.term); }
         )*
       ;
  
positive.code:       { $operator = new SumOp($SUM_OP); }
      | DIFFERENCE_OP
        { $operator = new DifferenceOp($DIFFERENCE_OP); }
      ;
  
  multiplicativeExpression returns [Term term]
  
negative.code:           $term = $exponentiationOperator.operator; }
          ee=exponentiationExpression
          { $exponentiationOperator.operator.setRightTerm($ee.term); }
        )?
      ;
  
  