code: func NewGraduatingExpirationProvider(initialExpiry, increments, maxExpiry time.Duration) ExpirationProvider {
  	var iteration uint32
  	return func() time.Duration {
  		expiry := initialExpiry + time.Duration(iteration)*increments
  		if expiry > maxExpiry {
  			return maxExpiry
  		}
  		iteration++
  		return expiry
  	}
  }
docs: // NewGraduatingExpirationProvider returns an expiration provider
  // that has an initial expiration and then expires in graduated increments
  // with a maximum expiration time.