My question was not well stated.  I apologize. 

I was trying to use the $toJSON() method on a %Persistent class and was getting an error.  However, the version of Caché is on 2016.1.0.  I have since learned that this is available on 2016.2 and forward.

Thank you Stefan. 

Great article!  Thank you for publishing.

Is multiple inheritance for the new %Object and %Array classes supported for existing classes that use %Library.Persistent?