title: Latex: how break a line when summing over two things
body: <p>Lets say I want to sum over <code>i \in S</code> and <code>i \in T</code>. Currently I use:</p>
  
  <pre><code>\displaystyle \sum_{i \in S, i \in T} i
  </code></pre>
  
  <p>But this will display the sum-overs one <strong>after</strong> the other and not one <strong>above</strong> the other like I want.</p>
  
  <p>How can I do this?</p>
  
  <p>thanks</p>