> unless Saxon performs some smart optimization.  What do you think ?

even if you don't use a key saxon (perhaps  just in the -sa version, I
can't remember) may decide you use some //foo[@x='aa'] idiom often
enough to internally use a key anyway. I tend to think of for-each group
as a sane syntax added at xslt2 for the muenchian grouping idiom that
Dimitre showed, which works well, but only looks normal as its posted
once a week for over half a decade to this list:-). No idea if the
internal implementations are similar though but that's processor
specific detail...

David