void RotateMatrices(
 IEnumerable<IEnumerable<Matrix>> collections,
 float degrees)
{
  var schPair = new ConcurrentExclusiveSchedulerPair(
   TaskScheduler.Default,
   maxConcurrencyLevel: 8);
  var scheduler = schPair.ConcurrentScheduler;
  var opts = new ParallelOptions { 
   TaskScheduler = scheduler };

  Parallel.ForEach(
   collections,
   opts,
   matrices => Parallel.ForEach(
    matrices,
    opts,
    m => m.Rotate(degrees)));
}