code: public JMFFieldDef getFieldDef(int accessor) {
      if (accessor < 0)
        return null;
      if (accessor < fields.length)
        return fields[accessor];
      accessor -= fields.length;
      if (accessor < variants.length)
        return variants[accessor];
      accessor -= variants.length;
      if (accessor >= boxed.length)
        return null;
      return ((JSVariant)fields[boxed[accessor][0]]).getBoxed().getFieldDef(boxed[accessor][1]);
    }
docs: Get a JSField node according to its accessor index.  Note that this method can
  return a variant box if it is presented with its accessor: this capability is used
  internally but layers above the JMFMessageData interface always present accessor
  indices corresponding to visible elements of the schema.