@Override
 public Vector apply(DenseVector a, SparseVector b) {
   Vector result = a.copy(factory);
   VectorIterator it = b.nonZeroIterator();
   while (it.hasNext()) {
     it.next();
     result.update(it.index(), Vectors.asMinusFunction(it.get()));
   }
   return result;
 }
 @Override
 public Vector apply(SparseVector a, DenseVector b) {
   Vector result = b.multiply(-1.0, factory);
   VectorIterator it = a.nonZeroIterator();
   while (it.hasNext()) {
     it.next();
     result.update(it.index(), Vectors.asPlusFunction(it.get()));
   }
   return result;
 }