arangosh> a1 = db.example.insert({ a : 1 });
{ 
  "_id" : "example/644371868", 
  "_rev" : "644371868", 
  "_key" : "644371868" 
}
arangosh> a2 = db._replace(a1, { a : 2 });
{ 
  "_id" : "example/644371868", 
  "_rev" : "644568476", 
  "_key" : "644371868" 
}
arangosh> a3 = db._replace(a1, { a : 3 });
[ArangoError 1200: precondition failed]