code: def start_group(self):
      self._grouping += 1
  
imports: from __future__ import with_statement
  from multiprocessing import Process, Queue, cpu_count
  from whoosh.compat import queue, xrange, pickle
  from whoosh.codec import base
  from whoosh.writing import SegmentWriter
  from whoosh.externalsort import imerge
  from whoosh.util import random_name
label_desc: Use image node_id 5 for calling the MpWriter obj's underlying member method code with example usage: obj.start_group() without return types