[fpc-pascal] how to define a generic subclass inheriting from another generic class?

[Dennis Poon]

What is the syntax. I could not find it in google.

e.g. in unit fgl,


generic TFPGMap<TKey, TData>=class(TFPSMap)
....
end;


How do I define a subclass TMyMap inheriting from TFPGMap without first 
specializing it?

I assumed

generic TMyMap <TKey, TData> = class(generic TFPGMap<TKey, TData>)
...
end

but fpc 2.6.2 did not accept this.

Please help.

Dennis