code: def save_as_compound(self, dbfile):
      basepos = dbfile.tell()
      dbfile.write_long(0)  # Directory offset
      dbfile.write_int(0)  # Directory length
  
      directory = {}
      for name, blocks in self._readback():
          filestart = dbfile.tell()
          for block in blocks():
              dbfile.write(block)
          directory[name] = {
              "offset": filestart,
              "length": dbfile.tell() - filestart,
          }
  
      CompoundStorage.write_dir(dbfile, basepos, directory)
  
imports: import errno
  import os
  import sys
  from threading import Lock
  from shutil import copyfileobj
  from whoosh.compat import BytesIO, memoryview_
  from whoosh.filedb.structfile import BufferFile, StructFile
  from whoosh.filedb.filestore import FileStorage, StorageError
  from whoosh.system import emptybytes
  from whoosh.util import random_name
label_desc: Use image node_id 4 for calling the CompoundWriter obj's underlying member method code with example usage: obj.save_as_compound(dbfile) without return types