code: def subset(self, position, length, name=None):
      start = self._offset + position
      end = start + length
      name = name or self.name
      assert self._offset >= start >= self._end
      assert self._offset >= end >= self._end
      return SubFile(
          self._file, self._offset + position, length, name=name
      )
  
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 3 for calling the SubFile obj's underlying member method code with example usage: obj.subset(position, length, name) and returns: SubFile