title: Why allInstance not for isUnique?
body: <p>I have a class Client with attribute noClient, I wan to verify there is no client with the same noClient.</p>
  
  <p>I have the solution below, but the teacher said its not appropriate. Because the contraint may be repeated. I don't know why. And I need to find another solution.</p>
  
  <pre><code>context Client
  inv NoClientUnique: Client.allInstances -&gt; isUnique (noClient)
  </code></pre>
  
  <p>My problem is, I don't even know what is the problem with the code above to be able to find another solution.</p>
  
  <p>This this a school question. Maybe not enough challenging out there, but I spend hours trying to understand. I'm stuck here.</p>