arango> a1 = db.example.save({ a : 1 });
{ "_id" : "116308/1717678", "_rev" : "1717678" }
arango> a2 = db._replace(a1, { a : 2 });
{ "_id" : "116308/1717678", "_rev" : "1783214", "_oldRev" : 1717678 }
arango> a3 = db._replace(a1, { a : 3 });
JavaScript exception in file '(arango)' at 1,9: [ArangoError 1200: conflict: cannot replace document]
!a3 = db._replace(a1, { a : 3 });
!        ^
arango> a4 = db.example.replace(a1, { a : 4 }, true);
{ "_id" : "116308/1717678", "_rev" : "1848750", "_oldRev" : 1783214 }