My read is that in both these cases ( document ordering and sibling access) that you would only have access to the information if you had a reference to the parent (or sequence).

 

Sequence order :

 

sequence [ A ,  1 , B , 2 ]

 

 

Given the sequence we know the order A,1,B,2

 

But given just the "B"  (say it is passed to a function) ... I dont think it would have a sibling axis like

 

  function x( B ) {

    B/previous-sibling::

}

 

 

x(  sequence[ 3 ] )

 

---- > how does fucntion x work ? I dont think it can.

    <child1  [ A, B  ] >

   <child2   [ C , D ] >

<root>

 

 

         create a sequence   [  child2/D , child1/A ]

 

         Order it in document order .... [ CRASH ]  ... I dont think you can because A and D dont have reference to their parents so dont know which come first when pulled out of the tree.