docs: def subscribe(hub, callback_url, topic_url, lease_seconds, secret,
                endpoint_hook_data):
      """5.2 Subscription Validation"""
  
      for validate in hub.validators:
          error = validate(callback_url, topic_url, lease_seconds, secret,
                           endpoint_hook_data)
          if error:
              send_denied(hub, callback_url, topic_url, error)
              return
  
      if intent_verified(hub, callback_url, 'subscribe', topic_url,
                         lease_seconds):
          hub.storage[topic_url, callback_url] = {
              'lease_seconds': lease_seconds,
              'secret': secret,
          }