appearance attribute for ui xml missing but present in examples code

In the GXT 3.1 examples there is a appearance attribute in AccordionLayout(UiBinder) demo. Here the screen shot:

Screen_Shot_2014-04-14_at_2.44.01_PM.jpg

I thought that the setAppearance method has been introduced with 3.1 beta but after trying to use the appearance attribute in my ui xml file, I always get an error saying:

Code:

Class ContentPanel has no appropriate setAppearance() method: <gxt:ContentPanel appearance='{contentPanelAppearance}' ui:field='messagePanel'> (:58)

Can anyone explain why is the appearance there in examples but is still missing in 3.1 beta jars?