public int compare(final ReadTransformer r1, final ReadTransformer r2) {
   if (r1.getOrderingConstraint() == r2.getOrderingConstraint()) return 0;
   return (r1.getOrderingConstraint() == OrderingConstraint.MUST_BE_FIRST
           || r2.getOrderingConstraint() == OrderingConstraint.MUST_BE_LAST)
       ? -1
       : 1;
 }