Problems using Interface with library objects

I was trying to set up library movieclips to extend an abstract class so they could share common methods, but can’t, as all library movieclip objects have to extend MovieClip.

My second thought was then to at least insure that all of these objects - which each need their own class now – implement the same functions – so I have the following

public class MyClass extends MovieClip implements MyInterface

and then in MyClass I have a function that implements someFunction in MyInterface.

I keep getting error 1044: Interface method someFunction in namespace MyInterface not implemented by class MyClass

what gives?

Thanks for any help