code: public T _get(long seqno) {
          lock.lock();
          try {
              int row_index=computeRow(seqno);
              if(row_index < 0 || row_index >= matrix.length)
                  return null;
              T[] row=matrix[row_index];
              if(row == null)
                  return null;
              int index=computeIndex(seqno);
              return index >= 0? row[index] : null;
          }
          finally {
              lock.unlock();
          }
      }
docs: To be used only for testing; doesn't do any index or sanity checks
  @param seqno
  @return