title: Check if swift array does not contain object
body: <p>I'm trying to filter a list of users from a Firebase query so that I only get users that are NOT listed in one of two arrays. The code I'm using doesn't work:</p>
  
  <pre><code>   let users = snapshot.childSnapshots.map {
                  User(snapshot: $0)
              }.filter{
                  guardiansArray.contains($0.key) == false || dependentsArray.contains($0.key) == false
          }
  </code></pre>
  
  <p>If I remove the <code>==false</code> code, I get the opposite effect of what I want: I get a list of users that ARE in either of the two arrays. How can I get the reverse effect?</p>
  
  <p>Thanks!</p>