code: func (g *GenericPLEG) Healthy() (bool, error) {
  	relistTime := g.getRelistTime()
  	if relistTime.IsZero() {
  		return false, fmt.Errorf("pleg has yet to be successful")
  	}
  	elapsed := g.clock.Since(relistTime)
  	if elapsed > relistThreshold {
  		return false, fmt.Errorf("pleg was last seen active %v ago; threshold is %v", elapsed, relistThreshold)
  	}
  	return true, nil
  }
docs: // Healthy check if PLEG work properly.
  // relistThreshold is the maximum interval between two relist.