arango> db.examples.ensureCapConstraint(10);
{ "id" : "147879/934311", "type" : "cap", "size" : 10, "isNewlyCreated" : true }

arango> for (var i = 0;  i < 20;  ++i) { var d = db.examples.save( { n : i } ); }

arango> db.examples.count();
10