title: how to update element in array using different update document for different arrayFilters
body: <p>suppose the document below:</p>
  
  <pre><code>{
       itemList:[
          {id:20,num:12}，
          {id:1,num:10}，
          {id:2,num:12}，
          {id:12,num:12}]，
       flowerId:0
  }
  </code></pre>
  
  <p>how did i update the element with id =1 to make its num decrease 2 and the element with id =2 to make its num decrease 3 in a single statement?</p>
  
  <p>here is my try:</p>
  
  <pre><code> db.getCollection('test').update({flowerId:0},
    {$inc:{"itemList.$[elem1].num":-2},$inc:{"itemList.$[elem2].num":-3}},
    {arrayFilters:[{"elem1.id":1},{"elem2.id":2}]}
    )
  </code></pre>
  
  <p>but it just print:</p>
  
  <pre><code>No array filter found for identifier 'elem2' in path 'itemList.$[elem2].num'
  </code></pre>
  
  <p>how can i do?</p>