def to_serializable
      serializable = super
      unless @proposer.nil?
        serializable['user1'] = @proposer.to_serializable
      end
      unless @proposed.nil?
        serializable['user2'] = @proposed.to_serializable
      end
      serializable['publicvisible'] = @privacy_level
      serializable
    end
def to_serializable
      serializable = super
      unless @proposer.nil?
        serializable['user1'] = @proposer.to_serializable
      end
      unless @proposed.nil?
        serializable['user2'] = @proposed.to_serializable
      end
      serializable['publicvisible'] = @privacy_level
      serializable
    end
:category: Serialization

 Returns a serializable representation, which only contains simple data
 types.
 The hash has the keys:
 * uri:: The URI.
 * id:: The identifier.
 * user1:: A User serializable of the proposer.
 * user2:: A User serializable of the proposed.
 * publicvisible:: The privacy level, see PrivacyLevel constants.