[Haskell-cafe] Importance of identity element in finger trees?

Hi all, I was looking at finger trees and the tricks for getting priority queues out of them seemed a little hackish, with a distinguished infinity element or maxBound. But it seems (although I have not yet tried it) like in many cases the monoid's identity element wouldn't be necessary (a bit like the difference between fold* and fold*1). Could a finger tree be applied to an arbitrary semigroup? Or is the identity more fundamental than it looks? Thanks, Dan