code: private void parseAdditiveExpr() throws TTXPathException {
  
          parseMultiplicativeExpr();
  
          String op = mToken.getContent();
          while (is(TokenType.PLUS, true) || is(TokenType.MINUS, true)) {
  
              // identify current operator kind
              // for (Operators op : Operators.values()) {
              // if (is(op.getOpName(), true)) {
  
              // parse second operand axis
  
              mPipeBuilder.addExpressionSingle();
  
              parseMultiplicativeExpr();
  
              mPipeBuilder.addOperatorExpression(getTransaction(), op);
              op = mToken.getContent();
          }
          // }
          // }
      }
docs: Parses the the rule AdditiveExpr according to the following production
  rule:
  <p>
  [12] AdditiveExpr ::= MultiplicativeExpr(("+" | "-") MultiplicativeExpr)* .
  </p>
  
  @throws TTXPathException