title: add a new value to array while keeping existing one
body: <p>I have below an example of data:</p>
  
  <pre><code>{
          "id": "2",
          "items":
              {
                "3" : { "blocks" : { "3" : { "txt" : 'xx' } } },
                "4" : { "blocks" : { "1" : { "txt" : 'yy'}, "2" : { "txt" : 'zz'} } }
              }
           }
  </code></pre>
  
  <p>I want to make it so that it looks like the below example data. Simply append a new value to items.3.blocks.3.txt while keeping existing value of it:</p>
  
  <pre><code> {
          "id": "2",
          "items":
              {
                "3" : { "blocks" : { "3" : { "txt" : 'xx, tt' } } },
                "4" : { "blocks" : { "1" : { "txt" : 'yy'}, "2" : { "txt" : 'zz'} } }
              }
           }
  </code></pre>
  
  <p>I run below but did not make any difference</p>
  
  <pre><code>dbx.test.update({"_id": ObjectId("5192264c02a03e374e67d7be")}, {'$addToSet': {'items.3.blocks.0.txt': 'tt'}}, )
  </code></pre>
  
  <p>what should be correct syntax, any help is appreciated...
  regards</p>