arangosh> a1 = db.example.insert({"a" : 1});
{ 
  "error" : false, 
  "_id" : "example/466773442", 
  "_rev" : "466773442", 
  "_key" : "466773442" 
}
arangosh> a2 = db.example.update(a1, {"b" : 2, "c" : 3});
{ 
  "error" : false, 
  "_id" : "example/466773442", 
  "_rev" : "467166658", 
  "_key" : "466773442" 
}
arangosh> a3 = db.example.update(a1, {"d" : 4});
[ArangoError 1200: precondition failed]
arangosh> a4 = db.example.update(a2, {"e" : 5, "f" : 6 });
{ 
  "error" : false, 
  "_id" : "example/466773442", 
  "_rev" : "467822018", 
  "_key" : "466773442" 
}
arangosh> db.example.document(a4);
{ 
  "a" : 1, 
  "b" : 2, 
  "c" : 3, 
  "e" : 5, 
  "f" : 6, 
  "_id" : "example/466773442", 
  "_rev" : "467822018", 
  "_key" : "466773442" 
}
arangosh> a5 = db.example.update(a4, {"a" : 1, c : 9, e : 42 });
{ 
  "error" : false, 
  "_id" : "example/466773442", 
  "_rev" : "468084162", 
  "_key" : "466773442" 
}
arangosh> db.example.document(a5);
{ 
  "a" : 1, 
  "b" : 2, 
  "c" : 9, 
  "e" : 42, 
  "f" : 6, 
  "_id" : "example/466773442", 
  "_rev" : "468084162", 
  "_key" : "466773442" 
}