code: public final CellName clusteringKey(ColumnFamily columnFamily) {
          Iterator<Cell> iterator = columnFamily.iterator();
          return iterator.hasNext() ? clusteringKey(iterator.next().name()) : null;
      }
docs: Returns the first clustering key contained in the specified {@link ColumnFamily}. Note that there could be more
  clustering keys in the column family.
  
  @param columnFamily A column family.
  @return The first clustering key contained in the specified {@link ColumnFamily}.