arangosh> a1 = db.example.insert({ a : 1 });
{ 
  "_id" : "example/500747507", 
  "_rev" : "500747507", 
  "_key" : "500747507" 
}
arangosh> a2 = db._replace(a1, { a : 2 });
{ 
  "_id" : "example/500747507", 
  "_rev" : "500944115", 
  "_key" : "500747507" 
}
arangosh> a3 = db._replace(a1, { a : 3 });
[ArangoError 1200: precondition failed]