docs: public void readExternal(ObjectInput in) throws IOException,
                                              ClassNotFoundException {
          super.readExternal( in );
          sink = (LeftTupleSinkPropagator) in.readObject();
          leftDeclaredMask = (BitMask) in.readObject();
          leftInferredMask = (BitMask) in.readObject();
          leftNegativeMask = (BitMask) in.readObject();
          positionInPath = in.readInt();
      }