> Assuming a large input, your approach looks more efficient to 
> me as it avoids grouping where indexing into the list does the job.
> 
> Now I guess from previous answers on this list given to 
> similar questions that this is all implementation-defined.
> 
> In spite of this, I'm asking whether that is all that can be 
> said here or whether there is a rationale here to favor 
> indexing over grouping when (a) processing time or (b) memory 
> consumption are important?

In Saxon (a) grouping will actually build a hash table in memory, so it uses
more memory, and (b) indexing in the form $seq[integer] is a constant-time
operation, so the indexing approach should definitely be better.

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