title: How to add a constraint that choose one tuple from a set?
body: <p>If I have a tuple like this</p>
  
  <pre><code>tuple index{
  int i;
  int j;
  int k;
  }
  {index}indexs=
  {
  &lt;1 1 1&gt;&lt;1 1 2&gt; &lt;1 2 1&gt; &lt;1 2 3&gt; &lt;2 1 2&gt; &lt;2 1 4&gt;
  }
  </code></pre>
  
  <p>How can I add a constraint that if i and j are the same, choose a tuple from it?</p>
  
  <p>Like, forall(i,j) sum(k in <strong>??</strong>) <code>x[i][j][k]=1</code></p>
  
  <p>Which means, choose one tuple from &lt;1 1 1> and&lt;1 1 2>, one tuple from &lt;1 2 1> &lt;1 2 2> &lt;1 2 3> and one tuple from &lt;2 1 2> &lt;2 1 4>?</p>