> db.runCommand({group: {ns: 'foo', '$reduce': "function(doc, prev)

{ prev.count += 1; }

", initial:

{count: new NumberLong(0)}

>
{
"retval" : [

{ "count" : "[object Object]11" }

],
"count" : 2,
"keys" : 1,
"ok" : 1
}