tachTombstoneBuffer() {
    if (!isPreTombstone()) {
      throw new AssertionError("Bug: only PRE_TOMBSTONES could detach buffer. " + this);
    }
    final ByteBuffer buffer = this.data;
    if (buffer == null) {
      throw new AssertionError("Bug: buffer already detached, .data is null. " + this);
    }
    this.data = null;
    return buffer;
  }


  private static 
tachTombstoneBuffer() {
    if (!isPreTom