let build_descriptor ops =
  let ops = sort_ops ops in        (* sort by id to prevent deadlock *)
  validate_distinct ops;           (* reject duplicate targets *)
  let desc = {
    status  = Atomic.make Active;
    words   = [||];
    retired = Atomic.make false
  } in
  let words =
    Array.of_list (
      List.map (fun (CAS (r, expected, desired)) -> {
        address   = r.cell;
        old_value = Obj.repr expected;
        new_value = Obj.repr desired;
        parent    = desc;             (* back-link to parent *)
      }) ops)
  in
  desc.words <- words;
  desc

let mcas ops =
  let desc = build_descriptor ops in
  mcas_desc desc   (* returns true on commit, false on abort *)