compatible extension of invoice following the wd-ubl-cmsc-cmguidelines-1.0

[Bryan Rasmussen <brs@itst.dk>]

Hey 
I have to do some compatible extensions to the UBL invoice. Here is my
understanding of what that entails:

The process should define a new namespace, this is fine as it dovetails with
our requirements. But just in case, is it allowed that my targetNamespace is
one of the UBL namespaces and I just include the relevant namespace, and do
extensions restrictions directly to the namespace? 

my next question is, in the context of having a new namespace:

If I want to extend PaymentMeans this means that I will need to have an
{efaktura}PaymentMeans, and inside of that {efaktura}PaymentMeans I have the
normal PaymentMeans structure, as well as my additions, basically requiring
that my  extensions extend all the way up the tree if I want to just extend
a leaf?