code: public final CellName clusteringKey(Document document) {
          String string = document.get(FIELD_NAME);
          ByteBuffer bb = ByteBufferUtils.fromString(string);
          return cellNameType.cellFromByteBuffer(bb);
      }
docs: Returns the clustering key contained in the specified {@link CellName}.
  
  @param document A {@link Document}.
  @return The clustering key contained in the specified {@link CellName}.