@Override
  public KnownSizeRecordWriter writerForEntryHeader(@NotNull K key) throws IOException {
    KnownSizeRecordWriter keyWriter = keyDescriptor.writerFor(key);
    int keySize = keyWriter.recordSize();
    if (keySize < 0) {
      throw new AssertionError("keySize(" + key + ")=" + keySize + ": must be strictly positive");
    }

    return new NonNullValueEntryWriter( keyWriter, /*valueWriter: */ KnownSizeRecordWriter.NOTHING);
  }
@Override
  public KnownSizeRecordWriter writerForEntryHeader(@NotNull K key)