code: public void addItem(@Nonnull final ExpressionItem item) {
      if (item == null) {
        throw new PreprocessorException("[Expression]Item is null", this.sources, this.includeStack, null);
      }
  
      if (last.isEmptySlot()) {
        last = new ExpressionTreeElement(item, this.includeStack, this.sources);
      } else {
        last = last.addTreeElement(new ExpressionTreeElement(item, this.includeStack, this.sources));
      }
    }
docs: Add new expression item into tree
  
  @param item an item to be added, must not be null