signature AC_Sort =
sig

  include Abbrev
  val sort : {assoc : thm, comm : thm,
              dest : term -> term * term, mk : term * term -> term,
              cmp : term * term -> order,
              combine : conv, preprocess : conv} -> conv

end

(*

   [sort {assoc,comm,dest,mk,cmp,combine,preprocess}] is a conversion for
   sorting terms with respect to an associative and commutative operator.
   It uses a merge sort internally, so should be reasonably efficient.

   The record's fields are:

     assoc:      associativity theorem in standard r-to-l format:
                  a + (b + c) = (a + b) + c
                 can be universally quantified

     comm:       commutativity theorem (can be universally quantified)

     dest:       destructor function for operator

     mk:         constructor function for operator

     cmp:        comparison function for performing sort.  Terms identified
                 as EQUAL will be combined by combine conversion.

     combine:    conv taking terms of the form (t1 op t2) where t1 and t2
                 have compared as equal.  Should always succeed (can be
                 ALL_CONV).

     preprocess: applied to all leaf terms as term is first examined.
                 If it fails or raises UNCHANGED (i.e., both ALL_CONV and
                 NO_CONV are OK here), nothing further is done.  If it
                 succeeds, further processing is performed on the resulting
                 term

    E.g., combine can combine numeric literals; preprocess could convert
    a - b into a + -b, or -x into -1 * x, or ~~p into p.

*)