arango> a1 = db.example.save({ a : 1 });
{ "error" : false, "_id" : "example/19660691", "_key" : "19660691", "_rev" : "19660691" }
arango> a2 = db._update(a1, { b : 2 });
{ "error" : false, "_id" : "example/19660691", "_key" : "19660691", "_rev" : "19791763" }
arango> a3 = db._update(a1, { c : 3 });
JavaScript exception in file 'client/client.js' at 2364,13: [ArangoError 1200: precondition failed]
!      throw new ArangoError(requestResult);
!            ^
arango> a4 = db.example.update(a1, { c : 3 }, true);
{ "error" : false, "_id" : "example/19660691", "_key" : "19660691", "_rev" : "19857299" }