code: private void mergeReturnElement(final Element stackElement) {
          if (returnElement != null)
              stackElement.merge(returnElement);
          returnElement = stackElement;
      }
docs: Merges the {@code returnElement} with the given element which was popped from the stack.
  If the {@code returnElement} existed before, the values are merged.
  
  @param stackElement The popped element