Thank you very much for the quick and thorough answers, the essence of
them being for my understanding:

1. Node sets are ordered.
2. The order is defined: <http://www.w3.org/TR/xpath#dt-document-order>
3. Because of the specific definition of/in (2), the "attributes-before-
children" requirement is met in my original post's second identity
transform example.
4. Seeing clearly again. :-)

Regards, Christian.