>   I don't think so.  Personnaly, I use the three-parameters 
> version of key() only when I need a subtree, not a document.
> If I just need to set the context node for the calling point, 
> I use instead:
> 
>     $input/key('things-by-a', .)
> 
>   But I guess it is a matter of taste.

I tend to use the 3-arg form. It's not just a matter of taste,

key('things-by-a', ., $input)

doesn't mean the same as

$input/key('things-by-a', .)

because the context node "." is different in the two cases, which will often
affect the evaluation of the second argument.

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