The specification of sort! reads:

   Function: sort! sequence less?
   Function: sort! sequence less? key

       Returns list, array, vector, or string sequence which has been
   mutated to order its elements according to less?. Given valid
   arguments, it is always the case that:

       (sorted? (sort! sequence less?) less?) => #t

Would be possible to add that the sorting is in-place?
That is guarantee, that no (or very little) extra memory
is allocated during the sorting process.

--
Jens Axel Søgaard