arangosh> a1 = db.example.insert({ a : 1 });
{ 
  "_id" : "example/1157755787", 
  "_rev" : "1157755787", 
  "_key" : "1157755787" 
}
arangosh> db._remove(a1);
true
arangosh> db._remove(a1);
[ArangoError 1202: document not found]
arangosh> db._remove(a1, true);
false