Didn't know that stuff, cool, thanks. (In early stage of learning F#). That feature you mention sounds a bit like Ruby's open classes, where, in file B (or even in file A), you can add a method to a class C (where C is defined in file A), outside of the definition of C. I was just reading somewhere else recently that some other language also has this feature, maybe it was about Lisp, in the recent thread about it.