code: func (conf *IndexWriterConfig) SetInfoStream(infoStream util.InfoStream) *IndexWriterConfig {
  	assert2(infoStream != nil, "Cannot set InfoStream implementation to null. "+
  		"To disable logging use InfoStream.NO_OUTPUT")
  	conf.infoStream = infoStream
  	return conf
  }
docs: // L523
  /*
  Information about merges, deletes and a message when maxFieldLength
  is reached will be printed to this. Must not be nil, but NO_OUTPUT
  may be used to surpress output.
  */