def compat_explain(cur):
    """
    Simulate MongoDB 3.0 explain result on prior versions.
    http://docs.mongodb.org/v3.0/reference/explain-results/
    """
    res = cur.explain()
    if 'nscannedObjects' in res:
        res['executionStats'] = dict(nReturned=res.pop('n'),
            totalKeysExamined=res.pop('nscanned'), totalDocsExamined=res.
            pop('nscannedObjects'), executionTimeMillis=res.pop('millis'))
    return res
