Name: ballot
Type: request
Purpose: >
  A ballot containing votes for a particular voting
Response: ballot-receipt
Inherits: message
Elements:
  From: { required: true }
  Date: null
  Event-Id: { required: true, type: identifier, class: event }
  Voting-Id: { required: true, type: identifier, class: voting }
  Votes: { required: true, type: list, max_count: 100, item_type: object, item_class: vote }
  Comment: { type: string, max_length: 1024 }
Rejection-Codes:
  31: Sender is not entitled to participate in this voting
  41: Unknown voting
  42: Voting has not been started yet
  43: Voting has already been closed
  95: Ballot contains invalid option
  96: Mandatory option is missing in ballot
  97: Ballot contains invalid comment
  98: Comment is too long