code: private long beforeAppend(DecoratedKey decoratedKey)
      {
          assert decoratedKey != null : "Keys must not be null"; // empty keys ARE allowed b/c of indexed column values
          if (lastWrittenKey != null && lastWrittenKey.compareTo(decoratedKey) >= 0)
              throw new RuntimeException("Last written key " + lastWrittenKey + " >= current key " + decoratedKey + " writing into " + getFilename());
          return (lastWrittenKey == null) ? 0 : dataFile.getFilePointer();
      }
docs: Perform sanity checks on @param decoratedKey and @return the position in the data file before any data is written