code: func (whisper *Whisper) SetMinimumPoW(val float64) error {
  	if val < 0.0 {
  		return fmt.Errorf("invalid PoW: %f", val)
  	}
  
  	whisper.settings.Store(minPowIdx, val)
  	whisper.notifyPeersAboutPowRequirementChange(val)
  
  	go func() {
  		// allow some time before all the peers have processed the notification
  		time.Sleep(time.Duration(whisper.syncAllowance) * time.Second)
  		whisper.settings.Store(minPowToleranceIdx, val)
  	}()
  
  	return nil
  }
docs: // SetMinimumPoW sets the minimal PoW required by this node