arangosh> a1 = db.example.insert({ a : 1 });
{ 
  "error" : false, 
  "_id" : "example/313993379", 
  "_rev" : "313993379", 
  "_key" : "313993379" 
}
arangosh> db._remove(a1);
true
arangosh> db._remove(a1);
[ArangoError 1202: document /_api/document/example/313993379 not found]
arangosh> db._remove(a1, true);
false