code: func (conf *IndexWriterConfig) SetMergeScheduler(mergeScheduler MergeScheduler) *IndexWriterConfig {
  	assert2(mergeScheduler != nil, "mergeScheduler must not be nil")
  	conf.mergeScheduler = mergeScheduler
  	return conf
  }
docs: /*
  Expert: sets the merge scheduler used by this writer. The default is
  ConcurentMergeScheduler.
  
  NOTE: the merge scheduler cannot be nil.
  
  Only takes effect when IndexWriter is first created.
  */