arangosh> a1 = db.example.insert({ a : 1 });
{ 
  "_id" : "example/626218396", 
  "_rev" : "626218396", 
  "_key" : "626218396" 
}
arangosh> a2 = db._update(a1, { b : 2 });
{ 
  "_id" : "example/626218396", 
  "_rev" : "626546076", 
  "_key" : "626218396" 
}
arangosh> a3 = db._update(a1, { c : 3 });
[ArangoError 1200: precondition failed]