title: JOIN header row values across a row based on non-blank values in cells
body: <p>So I have two rows:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>ID</th>
  <th>TagDog</th>
  <th>TagCat</th>
  <th>TagChair</th>
  <th>TagArm</th>
  <th>Grouped Tags (need help with this)</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>1</td>
  <td></td>
  <td>TRUE</td>
  <td></td>
  <td>TRUE</td>
  <td><em><strong>TagDog,TagArm</strong></em></td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>Row 1 consists mainly of Tags, while rows 2+ are entries. This data ties ENTRIES to TAGS.</p>
  <p>What I'm needing to do is concatenate/join the tag names per entry. For example, look at the last column above.</p>
  <p>I suspect we could write a formula that would:</p>
  <ul>
  <li>Create an array of non-empty cells in the row. (IE: [2,4])</li>
  <li>Return it with the header row <code>A</code> (IE: [A2,A4])</li>
  <li>Then <code>join</code> them together by a comma</li>
  </ul>
  <p>But I am unsure how to write the formula, or if this is even the best approach.</p>