title: Check if new data's value already exists?
body: <p>I have some data that's structured like this:</p>
  
  <pre><code>users:
     0SWwfKinLNSTcNrfHROvG0gGIYm1: "Godkoala"
     1koVpFjqBoWrHqFEXtC0dkhjjOW2: "Ricky Ballby"
     2KGNwFZe3IP4ehf0RIcUPQHS6rr2: "TetrahedroN"
     2V1izMVUXgQTWPBXSS3V5A6HklF3: "Ponyta"
  </code></pre>
  
  <p>I want users to NOT be able to write if the new data's value matches any of the value's in the <code>users</code> section. In other words I want the values of this <code>users</code> section to be unique.</p>
  
  <p>I know to check if a key already exists I can do something like:</p>
  
  <pre><code>".write":  "data.val() == null"
  </code></pre>
  
  <p>But I'm wondering if there's a way to check if a value doesn't exist instead.</p>