Actor

 { 
 // Union field type 
can be only one of the following: 
 "user" 
 : 
 { 
 object (  User 
 
) 
 } 
 , 
 "anonymous" 
 : 
 { 
 object (  AnonymousUser 
 
) 
 } 
 , 
 "impersonation" 
 : 
 { 
 object (  Impersonation 
 
) 
 } 
 , 
 "system" 
 : 
 { 
 object (  SystemEvent 
 
) 
 } 
 , 
 "administrator" 
 : 
 { 
 object (  Administrator 
 
) 
 } 
 // End of list of possible types for union field type 
. 
 }