code: private ParseTree parseAdditiveExpression() {
      SourcePosition start = getTreeStartLocation();
      ParseTree left = parseMultiplicativeExpression();
      while (peekAdditiveOperator()) {
        Token operator = nextToken();
        ParseTree right = parseMultiplicativeExpression();
        left = new BinaryOperatorTree(getTreeLocation(start), left, operator, right);
      }
      return left;
    }
docs: 11.6 Additive Expression