> on the other hand $a is a document structurally identical to 
> the current document but it is a copy, so it takes time and 
> memory proportional to the size of the document to build, 

That would be true in a naive implementation...

Saxon builds a virtual copy in cases where a copy of a document or element
node is not immediately attached to a new parent or written to a final
result tree. The virtual copy is initially cheap to build in time and
memory, but takes a little longer than the original to navigate.

Michael Kay
http://www.saxonica.com/