code: TreeSet<TreeEntry> fillEntries(TableKelp table)
    {
      TreeSet<TreeEntry> set = new TreeSet<>();
      
      for (BlockTree block : _blocks) {
        block.fillEntries(table, set, block.getIndex());
      }
      
      return set;
    }
docs: /*
  @Override
  Page writeCheckpoint(TableKelp table,
  SegmentWriter sOut,
  long oldSequence,
  int saveLength,
  int saveTail,
  int saveSequence)
  throws IOException
  {
  WriteStream os = sOut.getOut();
  
  os.write(_minKey);
  os.write(_maxKey);
  
  TreeSet<TreeEntry> entrySet = fillEntries(table, saveTail);
  
  for (TreeEntry entry : entrySet) {
  if (! entry.isInsert()) {
  continue;
  }
  
  os.write(entry.getMinKey());
  os.write(entry.getMaxKey());
  BitsUtil.writeInt(os, entry.getPid());
  }
  
  return this;
  }