title: Using $exists with $redact with mongo aggregate?
body: <p>I am using the $redact operator:</p>
  
  <pre><code>{ $redact: {
      $cond: {
          "if": {
              "$lt": [ "$number1", "$number2" ],
          },
          "then": "$$KEEP",
          "else": "$$PRUNE"
      }
  }}
  </code></pre>
  
  <p>I would like to <code>$and: [{$lt:["$number1, $number2"]}, {$exists:[$number3, 1]}]</code>, but I cannot get the $exist, or checking for null.</p>