Name: membership-cancellation
Type: request
Purpose: >
  Cancel an identity's membership in a particular group
Response: membership-cancellation-confirmation
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Date: { required: true }
  Group-Id: { required: true, type: identifier, class: group }
  Participant-Id: { required: true, type: identity }