title: What does the "h" in oplog.rs document stand for?
body: <p>As in here:    </p>
  
  <pre><code>{
      "ts" : Timestamp(1374832131, 1),
      "h" : NumberLong("-1336944105039123379"),
      "v" : 2,
      "op" : "i",
      "ns" : "users.users",
      "o" : {
              "_id" : ObjectId("51f24603d98de3716b1db672")
      }
  }
  </code></pre>
  
  <p>Thanks</p>