code: public JSON with(TreeCodec c)
      {
          if (c == _treeCodec) {
              return this;
          }
          return _with(_features, _streamFactory, c,
                  _reader, _writer.with(c), _prettyPrinter);
      }
docs: Mutant factory for constructing an instance with specified {@link TreeCodec},
  and returning new instance (or, if there would be no change, this instance).