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  <title>Deeply Nested Collapsible List</title>
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    details {
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    ul {
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  <details>
    <summary>I.5</summary>
    <ul>
      <li>
        <details>
          <summary>I.3</summary>
          <ul>
            <li>
              <details>
                <summary>I.2</summary>
                <ul>
                  <li>I.1</li>
                </ul>
              </details>
            </li>
          </ul>
        </details>
      </li>
      <li>I.4</li>
    </ul>
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