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