arangosh> a1 = db.example.insert({ a : 1 });
{ 
  "error" : false, 
  "_id" : "example/1084253654", 
  "_rev" : "1084253654", 
  "_key" : "1084253654" 
}
arangosh> a2 = db._update(a1, { b : 2 });
{ 
  "error" : false, 
  "_id" : "example/1084253654", 
  "_rev" : "1084581334", 
  "_key" : "1084253654" 
}
arangosh> a3 = db._update(a1, { c : 3 });
[ArangoError 1200: precondition failed]