Generating classes on the fly

Hello folks

I am generating groovy classes (ClassNode objects) on the fly. I do this by
adding an AST transformation that will add these classes as inner classes
to other classes (that were fed to the compiler as source files). This
works. However, I want to generate classes on the fly without attaching
them to existing class nodes as inner classes. How do I achieve this? when
I use the method addClassNode in compilation unit, I get sourceUnit null
pointer exceptions (I need the sourceunit to lookup other AST customizers)

What is the right way to build a source unit that can be used for these
class nodes that I generate on the fly?

regards
Saravanan