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Introduction</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-2/c-b-character-syntax.html">2. Syntax</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-3/d-b-evaluation.html">3. Evaluation and Compilation</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-4/e-b-introduction.html">4. Types and Classes</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-5/f-b-generalized-reference.html">5. Data and Control Flow</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-6/g-b-the-loop-facility.html">6. Iteration</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-7/h-b-object-creation-and-initialization.html">7. Objects</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../category/81-structures-dictionary.html">8. Structures</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-9/j-b-condition-system-concepts.html">9. Conditions</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-10/ba-b-symbol-concepts.html">10. Symbols</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-11/bb-b-package-concepts.html">11. Packages</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-12/bc-b-number-concepts.html">12. Numbers</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-13/bd-b-character-concepts.html">13. Characters</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret menu__link--active" aria-expanded="true" href="be-b-cons-concepts.html">14. Conses</a></div><ul style="display:block;overflow:visible;height:auto" class="menu__list"><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link menu__link--active" aria-current="page" tabindex="0" href="be-b-cons-concepts.html">14.1 Cons Concepts</a></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-2 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist" aria-expanded="false" tabindex="0" href="../category/142-conses-dictionary.html">14.2 Conses Dictionary</a><button aria-label="Expand sidebar category '14.2 Conses Dictionary'" type="button" class="clean-btn menu__caret"></button></div></li></ul></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-15/bf-b-array-concepts.html">15. Arrays</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-16/bg-b-string-concepts.html">16. Strings</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-17/bh-b-sequence-concepts.html">17. Sequences</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-18/bi-b-hash-table-concepts.html">18. Hash Tables</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-19/bj-b-overview-of-filenames.html">19. Filenames</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-20/ca-b-file-system-concepts.html">20. Files</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-21/cb-b-stream-concepts.html">21. Streams</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-22/cc-b-the-lisp-printer.html">22. Printer</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-23/cd-b-reader-concepts.html">23. Reader</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-24/ce-b-system-construction-concepts.html">24. System Construction</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-25/cf-b-the-external-environment.html">25. External Environment</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../environments.html">26. Environments</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../meta-object-protocol.html">27. Meta Object Protocol</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../data-structures.html">28. Data Structures</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../portability/trivial-packages.html">29. Portability</a></div></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-1 menu__list-item"><a class="menu__link" href="../dictionary-entries.html">dictionary-entries</a></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-26/intro.html">Glossary</a></div></li></ul></nav></div></div></aside><main class="docMainContainer_TBSr"><div class="container padding-top--md padding-bottom--lg"><div class="row"><div class="col docItemCol_VOVn"><div class="docItemContainer_Djhp"><article><nav class="theme-doc-breadcrumbs breadcrumbsContainer_Z_bl" aria-label="Breadcrumbs"><ul class="breadcrumbs" itemscope="" itemtype="https://schema.org/BreadcrumbList"><li class="breadcrumbs__item"><a aria-label="Home page" class="breadcrumbs__link" href="../index.html"><svg viewBox="0 0 24 24" class="breadcrumbHomeIcon_YNFT"><path d="M10 19v-5h4v5c0 .55.45 1 1 1h3c.55 0 1-.45 1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill="currentColor"></path></svg></a></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">14. Conses</span><meta itemprop="position" content="1"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">14.1 Cons Concepts</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h1>14.1 Cons Concepts</h1>
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<p>A <span><i>cons</i></span> is a compound data <span><i>object</i></span> having two components called the <span><i>car</i></span> and the <span><i>cdr</i></span> .</p>
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<p>|</p><p><strong>car cons rplacd</strong> </p><p><strong>cdr rplaca</strong></p>|<p></p>
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<p>| :- |</p>
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<p><strong>Figure 14–1. Some defined names relating to conses.</strong></p>
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<p>Depending on context, a group of connected <em>conses</em> can be viewed in a variety of different ways. A variety of operations is provided to support each of these various views.</p>
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id="1411-conses-as-trees">14.1.1 Conses as Trees<a href="be-b-cons-concepts.html#1411-conses-as-trees" class="hash-link" aria-label="Direct link to 14.1.1 Conses as Trees" title="Direct link to 14.1.1 Conses as Trees"></a></h2>
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<p>A <span><i>tree</i></span> is a binary recursive data structure made up of <em>conses</em> and <span><i>atoms</i></span>: the <em>conses</em> are themselves also <span><i>trees</i></span> (sometimes called “subtrees” or “branches”), and the <span><i>atoms</i></span> are terminal nodes (sometimes called <em>leaves</em>). Typically, the <em>leaves</em> represent data while the branches establish some relationship among that data.</p>
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<p>|</p><p><strong>caaaar caddar cdar nsubst</strong> </p><p><strong>caaadr cadddr cddaar nsubst-if</strong> </p><p><strong>caaar caddr cddadr nsubst-if-not caadar cadr cddar nthcdr</strong> </p><p><strong>caaddr cdaaar cdddar sublis</strong> </p><p><strong>caadr cdaadr cddddr subst</strong> </p><p><strong>caar cdaar cdddr subst-if</strong> </p><p><strong>cadaar cdadar cddr subst-if-not cadadr cdaddr copy-tree tree-equal cadar cdadr nsublis</strong></p>|<p></p>
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<p>| :- |</p>
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<p><strong>Figure 14–2. Some defined names relating to trees.</strong></p>
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<h3 class="anchor anchorWithStickyNavbar_LWe7" id="14111-general-restrictions-on-parameters-that-must-be-trees">14.1.1.1 General Restrictions on Parameters that must be Trees<a href="be-b-cons-concepts.html#14111-general-restrictions-on-parameters-that-must-be-trees" class="hash-link" aria-label="Direct link to 14.1.1.1 General Restrictions on Parameters that must be Trees" title="Direct link to 14.1.1.1 General Restrictions on Parameters that must be Trees"></a></h3>
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<p>Except as explicitly stated otherwise, for any <em>standardized function</em> that takes a <span><i>parameter</i></span> that is required to be a <span><i>tree</i></span>, the consequences are undefined if that <span><i>tree</i></span> is circular.</p>
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id="1412-conses-as-lists">14.1.2 Conses as Lists<a href="be-b-cons-concepts.html#1412-conses-as-lists" class="hash-link" aria-label="Direct link to 14.1.2 Conses as Lists" title="Direct link to 14.1.2 Conses as Lists"></a></h2>
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<!-- -->
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<p>A <span><i>list</i></span> is a chain of <em>conses</em> in which the <span><i>car</i></span> of each <span><i>cons</i></span> is an <span><i>element</i></span> of the <span><i>list</i></span>, and the <span><i>cdr</i></span> of each <span><i>cons</i></span> is either the next link in the chain or a terminating <span><i>atom</i></span>.</p>
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<p>A <span><i>proper list</i></span> is a <span><i>list</i></span> terminated by the <span><i>empty list</i></span>. The <span><i>empty list</i></span> is a <span><i>proper list</i></span>, but is not a <span><i>cons</i></span>.</p>
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<p>An <em>improper list</em> is a <span><i>list</i></span> that is not a <span><i>proper list</i></span>; that is, it is a <span><i>circular list</i></span> or a <span><i>dotted list</i></span>.</p>
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<p>A <span><i>dotted list</i></span> is a <span><i>list</i></span> that has a terminating <span><i>atom</i></span> that is not the <span><i>empty list</i></span>. A <em>non-nil atom</em> by itself is not considered to be a <span><i>list</i></span> of any kind—not even a <span><i>dotted list</i></span>.</p>
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<p>A <span><i>circular list</i></span> is a chain of <em>conses</em> that has no termination because some <span><i>cons</i></span> in the chain is the <span><i>cdr</i></span> of a later <span><i>cons</i></span>.</p>
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<p>|</p><p><strong>append last nbutlast rest</strong> </p><p><strong>butlast ldiff nconc revappend copy-alist list ninth second copy-list list* nreconc seventh eighth list-length nth sixth endp make-list nthcdr tailp</strong> </p><p><strong>fifth member pop tenth first member-if push third fourth member-if-not pushnew</strong></p>|<p></p>
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<p>| :- |</p>
|
||
<p><strong>Figure 14–3. Some defined names relating to lists.</strong></p>
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<h3 class="anchor anchorWithStickyNavbar_LWe7" id="14121-lists-as-association-lists">14.1.2.1 Lists as Association Lists<a href="be-b-cons-concepts.html#14121-lists-as-association-lists" class="hash-link" aria-label="Direct link to 14.1.2.1 Lists as Association Lists" title="Direct link to 14.1.2.1 Lists as Association Lists"></a></h3>
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<!-- -->
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<p>An <span><i>association list</i></span> is a <span><i>list</i></span> of <em>conses</em> representing an association of <span><i>keys</i></span> with <span><i>values</i></span>, where the <span><i>car</i></span> of each <span><i>cons</i></span> is the <span><i>key</i></span> and the <span><i>cdr</i></span> is the <span><i>value</i></span> associated with that <span><i>key</i></span>.</p>
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<p>|</p><p><strong>acons assoc-if pairlis rassoc-if</strong> </p><p><strong>assoc assoc-if-not rassoc rassoc-if-not</strong></p>|<p></p>
|
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<p>| :- |</p>
|
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<p><strong>Figure 14–4. Some defined names related to assocation lists.</strong></p>
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<h3 class="anchor anchorWithStickyNavbar_LWe7" id="14122-lists-as-sets">14.1.2.2 Lists as Sets<a href="be-b-cons-concepts.html#14122-lists-as-sets" class="hash-link" aria-label="Direct link to 14.1.2.2 Lists as Sets" title="Direct link to 14.1.2.2 Lists as Sets"></a></h3>
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<!-- -->
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<p><span><i>Lists</i></span> are sometimes viewed as sets by considering their elements unordered and by assuming there is no duplication of elements.</p>
|
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<p>|</p><p><strong>adjoin nset-difference set-difference union intersection nset-exclusive-or set-exclusive-or</strong> </p><p><strong>nintersection nunion subsetp</strong></p>|<p></p>
|
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<p>| :- |</p>
|
||
<p><strong>Figure 14–5. Some defined names related to sets.</strong></p>
|
||
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="14123-general-restrictions-on-parameters-that-must-be-lists">14.1.2.3 General Restrictions on Parameters that must be Lists<a href="be-b-cons-concepts.html#14123-general-restrictions-on-parameters-that-must-be-lists" class="hash-link" aria-label="Direct link to 14.1.2.3 General Restrictions on Parameters that must be Lists" title="Direct link to 14.1.2.3 General Restrictions on Parameters that must be Lists"></a></h3>
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<!-- -->
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<!-- -->
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<p>Except as explicitly specified otherwise, any <em>standardized function</em> that takes a <span><i>parameter</i></span> that is required to be a <span><i>list</i></span> should be prepared to signal an error of <span><i>type</i></span> <span><b>type-error</b></span> if the <span><i>value</i></span> received is a <span><i>dotted list</i></span>.</p>
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<p>Except as explicitly specified otherwise, for any <em>standardized function</em> that takes a <span><i>parameter</i></span> that is required to be a <span><i>list</i></span>, the consequences are undefined if that <span><i>list</i></span> is <span><i>circular</i></span> .</p></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/lisp-docs/cl-language-reference/tree/main/docs/chap-14/be-b-cons-concepts.md" target="_blank" rel="noopener noreferrer" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_vwxv"><span class="theme-last-updated">Last updated<!-- --> by 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