title: MongoDB: error with $divide and $multiply
body: <p>I'm creating a MongoDB aggregation pipeline and I'm stuck at this stage:</p>
  
  <pre><code>        $group: {
              _id: {checkType: "$_id.checkType", resultCode: "$_id.resultCode"},
              count: { $sum: "$count" },
              ctv: { $sum: "$ctv" },
              perc:{$multiply:[{$divide:["$ctv","$count"]},100]},
              weight: { $divide: [ "$ctv", "$count"] },
              details: { $push: "$$ROOT" }
          }
  </code></pre>
  
  <p>It gives the error "The $multiply accumulator is a unary operator".  Similarly if I remove the line with $multiply I get "The $divide accumulator is a unary operator" on the subsequent line.  I cannot find a description for this error on the Net. What's wrong in my sintax?</p>