code: func (c *Client) Subscription(tag string) (*Subscription, error) {
  	subs, err := c.Subscriptions()
  	if err != nil {
  		return nil, err
  	}
  
  	for i := range subs {
  		if subs[i].Channel.Tag == tag {
  			subs[i].Client = c
  			return subs[i], nil
  		}
  	}
  	return nil, ErrDeviceNotFound
  }
docs: // Subscription fetches an subscription with a given channel tag from PushBullet.