code: protected JSONReader _with(int features,
              ValueReaderLocator td, TreeCodec tc, CollectionBuilder lb, MapBuilder mb)
      {
          if (getClass() != JSONReader.class) { // sanity check
              throw new IllegalStateException("Sub-classes MUST override _with(...)");
          }
          return new JSONReader(features, td, tc, lb, mb);
      }
docs: Overridable method that all mutant factories call if a new instance
  is to be constructed