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Why is JavaScript necessary?</a></li><li class="main-nav__course-contents-item main-nav__course-contents-item--current"><a href="/courses/intro-to-web-development/javascript/theme-switcher">2. Introduction to the theme switcher</a></li><li class="main-nav__course-contents-item"><a href="/courses/intro-to-web-development/javascript/query-selector">3. Find an element using querySelector</a></li><li class="main-nav__course-contents-item"><a href="/courses/intro-to-web-development/javascript/output-element">4. Output the element to the console.</a></li><li class="main-nav__course-contents-item"><a href="/courses/intro-to-web-development/javascript/remove-class">5. Let’s remove the class using classList.remove</a></li><li class="main-nav__course-contents-item"><a href="/courses/intro-to-web-development/javascript/add-class">6. 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fill="#EFA75B"/><path d="M31.963 50.968c-.197-.004-.358-.157-.372-.353-.018-.207.158-.378.364-.354 3.245-.26 3.505-2.156 3.608-2.25-.006-.19.183-.325.362-.26.193.01.35.16.363.354 0 .084 0 1.906 3.422 2 .192.008.348.16.362.353-.018.19-.172.34-.363.353-1.352.11-2.68-.4-3.61-1.386-.095-.096-.16-.22-.185-.353-.982 1.174-2.423 1.866-3.952 1.898z" fill="#151841"/><path d="M32.14 43.427c.186-.52 3.608.083 3.608.083s2.975-.688 3.422.084c-.05.288-.175.557-.363.78-.515-.056-1.037.004-1.525.178-.333.61-.488 1.304-.446 2-.222.394-.574.7-.995.864-.398-.112-.75-.354-.994-.688.047-.75-.146-1.496-.55-2.13-.27-.26-1.264.084-1.626-.083l-.53-1.088zm6.89.242c-.05.286-.175.556-.363.78-.503-.063-1.015-.012-1.497.148-.333.612-.487 1.305-.446 2-.222.394-.574.7-.995.864-.4-.112-.75-.354-.996-.688.053-.76-.14-1.515-.55-2.157-.268-.26-1.263.084-1.626-.084-.26-.23-.45-.53-.55-.864" fill="#CF4A27"/><path d="M35.748 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2.334.725-.13.837-1.46.93-2.13h.112c.937.492 1.647 1.33 1.98 2.334-.388.176-.793.31-1.21.4zm18.365-2.864c-1.104-.145-2.215-.232-3.33-.26-.546-.01-1.093.05-1.626.176-1.166.483-2.08 1.425-2.53 2.604l-.158.79 1.795.623c.558.153 1.132.24 1.71.26h.28c3.245-.177 4.137-3.72 4.137-3.72l.084-.353zm-1.264 1.86c-.54 1.394-2.882 1.646-2.966 1.646-.517-.02-1.03-.11-1.525-.26-.186-.085-.633-.26-1.08-.43.374-.966 1.117-1.746 2.065-2.165 0 .68.158 1.953.81 2.083.65.13.613-1.535.566-2.26.934.005 1.865.064 2.79.177-.11.316-.473.65-.66 1.17z" fill="#151841"/><path d="M19.26 39.27c.535 1.362 1.56 2.473 2.874 3.115.534.312 1.204-.07 1.207-.69.002-.285-.15-.55-.397-.695-.918-.46-1.65-1.222-2.073-2.158-.344-.957-1.972-.52-1.61.428zm29.45 3.125c1.083-.59 2.01-1.428 2.706-2.446.3-.556-.112-1.228-.743-1.21-.293.007-.56.17-.698.428-.568.776-1.306 1.412-2.158 1.86-.394.23-.517.742-.27 1.125.263.376.773.482 1.163.242z" fill="#EFA75B" stroke-width="13"/><path d="M16.025 32.436c1.525-1.386 2.427-4.408 2.79-6.835-.084.354 2.613.43 4.138-1.208 1.078-1.125.446-2-.186-3.375-1.71-3.552-4.585-6.323-7.384-8.918-.716-.69-1.44-2.428-2.706-2-.995.353-.93 3.2-.716 4.073 1.09 7.448 4.065 18.263 4.065 18.263zm37.372-.78c-1.704-1.826-2.822-4.12-3.208-6.585 0 .354-2.614.354-4.056-1.29-.995-1.127-.362-1.86.363-3.117 1.86-3.198 4.863-5.718 7.746-8.136.81-.604 1.627-1.47 2.79-.93.994.428.715 2.334.548 3.115-1.083 5.72-2.48 11.376-4.183 16.943z" fill="#FFF"/><path d="M53.564 20.422c-1.98.51-4.138 1.636-5.105 3.487-.43-2.196 1.645-4.65 3.393-5.832.333-.21.312-.703-.037-.886-.162-.085-.357-.076-.512.022-2.157 1.562-4.417 4.5-3.784 7.188.076.262.354.41.614.325l-.075.28c-.085.603.93.864.994.26.446-2.158 2.706-3.376 4.77-3.897.372-.076.522-.526.27-.81-.133-.15-.342-.204-.53-.138zM17.457 18.06c-.55-.354-.995.428-.55.865 1.433 1.004 3.182 3.05 3.395 4.956-.93-1.86-3.05-2.975-5.105-3.486-.386-.005-.633.41-.445.747.042.076.103.14.176.184 2.073.52 4.324 1.73 4.77 3.896.106.328.5.46.78.26.273.192.653.06.745-.26.735-2.66-1.6-5.728-3.766-7.16z" fill="#F1662A"/></svg></span><p class="course-loading-overlay__message"><b class="course-loading-overlay__message-title">Loading…</b><br><span class="course-loading-overlay__message-text">Everything will be ready in few seconds</span></p></div></div><div class="course-layout__sidebar course-theory"><div class="course-theory__inner course-theory__inner--left-stub"><button class="course-theory__close icon-close icon-close--light" type="button">Close</button><ul class="course-tabs course-tabs--non-interactive course-layout__theory-heading js--theory-tabs"><li class="course-tabs__item course-tabs__item--external course-tab course-tab--centered"><span class="course-tab__icon"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-theory"></use></svg></span><span class="course-tab__title">Theory</span></li><li class="course-tabs__item course-tabs__item--internal course-tab course-tab--centered course-tab--active js--theory-tab__theory"><span class="course-tab__icon"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-theory"></use></svg></span><span class="course-tab__title">Theory</span></li><li class="course-tabs__item course-tabs__item--internal course-tab course-tab--centered js--theory-tab__comments"><span class="course-tab__icon"><svg aria-hidden="true" style="width: 16px; height: 16px; fill: #2e2c9c;"><use xlink:href="/img/sprites/up.svg#comment"></use></svg></span><span class="course-tab__title">Comments</span></li></ul><section class="course-theory__content"><div class="js--theory-pane__theory course-theory__content-inner"><h1 class="course-theory__content-heading">Introduction to the theme switcher</h1><div class="course-theory__content-text"><blockquote><p>Meow! Why are you sitting around doing nothing? Get to work! The news will not wait!</p></blockquote><p>Meet Muffin the cat. He is the owner of his own web development studio. It was members of his team who created the news website whose main page we saw in the last assignment. It seems that Muffin mistook us for his developers. All humans look the same to him. Why not play along with him? After all, the most effective way of learning anything is to practice it. So, what is it that our bushy-tailed boss needs today?</p><p>The news site has two themes: light and dark. Our task is to create a feature that allows visitors to switch between them by clicking on the “Change Theme” button. In the future, users will be able to switch between these themes on any page of the site, but for now we will test our script on just the subscription page. All of the page contents can be found inside the element with the <code>page</code> class. The <code class="nowrap">light-theme</code> class is responsible for the light theme, whereas the <code class="nowrap">dark-theme</code> class is responsible for the dark one. In order to change themes, we need to remove one class from the <code>page</code> and add another.</p><p>Before we write a script, let’s manually switch the theme on the page. This will help us to better understand what we need to do. To do this, change the classes for the <code>page</code> element right in the HTML.</p></div></div><div class="js--theory-pane__comments hidden course-theory__content-inner"><div class="course-theory__content-text"><h2 class="course-theory__content-heading">Comments</h2><div id="disqus_thread"></div><script>
/**
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* LEARN WHY DEFINING THESE VARIABLES IS IMPORTANT: https://disqus.com/admin/universalcode/#configuration-variables */
var disqus_config = function () {
this.page.url = "https://htmlacademy.org/courses/intro-to-web-development/javascript/theme-switcher";
this.page.identifier = "task-307";
};
(function() { // DON'T EDIT BELOW THIS LINE
var d = document, s = d.createElement('script');
s.src = 'https://htmlacademy.disqus.com/embed.js';
s.setAttribute('data-timestamp', +new Date());
(d.head || d.body).appendChild(s);
})();
</script><noscript>Please enable JavaScript to view the <a href="https://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript></div></div></section><div class="course-theory__controls"><button class="button button--close" type="button">To the task</button></div></div></div><div class="course-layout__column course-layout__column--left"><ul class="js--course-tabs course-tabs course-layout__course-tabs"><li class="js--editor-tab course-tabs__item course-tab course-tabs__item--limited course-tabs__item--limited-large course-tab--active" data-editor="html"><span class="course-tab__title course-tab__title--indent"><span class="course-tab__title-part course-tab__title-part--active">index.html</span></span><button class="js--editor-split course-tab__button course-tab__button--split" type="button"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-split"></use></svg><span class="tooltip tooltip--light tooltip--to-top tooltip--with-wrap tooltip--short course-tab__tooltip"><span class="tooltip__wrap">Split mode</span></span></button></li><li class="js--editor-tab course-tabs__item course-tab course-tabs__item--limited course-tabs__item--limited-large" data-editor="css"><span class="course-tab__title course-tab__title--indent"><span class="course-tab__title-part course-tab__title-part--active">style.css</span></span><button class="js--editor-split course-tab__button course-tab__button--split" type="button"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-split"></use></svg><span class="tooltip tooltip--light tooltip--to-top tooltip--with-wrap tooltip--short course-tab__tooltip"><span class="tooltip__wrap">Split mode</span></span></button></li></ul><div class="course-layout__editors"><div class="js--editor course-editor course-layout__editor" id="" data-tab="html" data-split=""><span class="course-editor__label"><span class="course-editor__label-item">HTML</span><button class="course-editor__label-item course-editor__label-item--interactive hidden js--editor__unsplit-button course-editor__label-item--simple" title="No split" type="button"><svg width="7" height="10" viewBox="0 0 7 10" xmlns="http://www.w3.org/2000/svg"><g stroke="currentColor" fill="none" fill-rule="evenodd"><path stroke-linecap="round" d="M.6 3.503L3.443.518l2.842 2.985"></path><path d="M3.5 1.5v7.16" stroke-linecap="square"></path></g></svg></button></span><div class="course-editor__item" id="html-editor"><!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<link rel="stylesheet" href="setting.css">
<link rel="stylesheet" href="style.css">
<title>FlashNews!</title>
</head>
<body class="page light-theme">
<header class="page-header">
<div class="container">
<a class="header-link" href="#">Main page</a>
<button class="theme-button" type="button">Change the theme</button>
</div>
</header>
<main class="inner-main">
<div class="container">
<h1 class="inner-heading">Subscribe to our newsletter</h1>
<p class="subscription-message">We promise to send you news no more than once a day. We will only send you the most interesting and important stories.</p>
<form action="https://echo.htmlacademy.ru/courses" method="post" class="subscription">
<div class="subscription-inner">
<label class="subscription-label" for="subscription-email">email</label>
<input type="email" class="subscription-email" placeholder="muffin@flashnews.ru" value="" required id="subscription-email">
</div>
<button class="button subscription-button" type="submit">Subscribe</button>
</form>
<section class="news-list list-tiles-view">
<h2 class="news-list-heading">Or continue reading more news stories.</h2>
<article class="new-block">
<img src="img/new-graph.jpg" alt="New library">
<div class="new-block-text">
<h3>New graphics library</h3>
<p>Now you can create a dashboard in just a matter of seconds.</p>
<time datetime="2019-10-16">October 16, 2019</time>
</div>
</article>
<article class="new-block">
<img src="img/new-robot.jpg" alt="What is going on with robots?">
<div class="new-block-text">
<h3>What is going on with robots?</h3>
<p>A lot of interesting things are happening in robotics. This could be a piece of news about the latest developments, but that is not what this is.</p>
<time datetime="2019-10-15">October 15, 2019</time>
</div>
</article>
</section>
</div>
</main>
<footer class="page-footer">
<div class="container">
<p>© FlashNews!</p>
<a class="footer-logo">
<img src="img/white-logo.svg" alt="FlashNews! portal logo">
</a>
</div>
</footer>
</body>
</html>
</div></div><div class="js--editor course-editor course-layout__editor course-editor--hidden" id="" data-tab="css" data-split=""><span class="course-editor__label"><span class="course-editor__label-item">CSS</span><button class="course-editor__label-item course-editor__label-item--interactive hidden js--editor__unsplit-button course-editor__label-item--simple" title="No split" type="button"><svg width="7" height="10" viewBox="0 0 7 10" xmlns="http://www.w3.org/2000/svg"><g stroke="currentColor" fill="none" fill-rule="evenodd"><path stroke-linecap="round" d="M.6 3.503L3.443.518l2.842 2.985"></path><path d="M3.5 1.5v7.16" stroke-linecap="square"></path></g></svg></button></span><div class="course-editor__item" id="css-editor">/* Light theme */
.light-theme {
color: #333333;
background-color: #eae9f2;
}
.light-theme .page-header {
background-color: #ffffff;
box-shadow: 0 2px 2px rgba(0, 0, 0, 0.2);
}
.light-theme .header-link {
color: #6653d9;
}
.light-theme .header-link::before {
background: url("img/arrow-back-light.svg") no-repeat 0 0;
}
.light-theme .theme-button {
color: #6653d9;
border: 1px solid #6653d9;
}
.light-theme .theme-button::before {
background-image: url("img/moon-normal.svg");
}
.light-theme .theme-button:focus,
.light-theme .theme-button:hover {
color: #ffffff;
background-color: #6653d9;
}
.light-theme .theme-button:focus::before,
.light-theme .theme-button:hover::before {
background-image: url("img/moon-hover.svg");
}
.light-theme .menu-open {
background-color: #ffffff;
background-image: url("img/menu-open-light.svg");
}
.light-theme .menu-open:focus,
.light-theme .menu-open:hover {
background-color: #6653d9;
background-image: url("img/menu-open-dark.svg");
}
.light-theme .menu-close {
background-color: #6653d9;
}
.light-theme .menu-close:focus,
.light-theme .menu-close:hover {
background-color: #473c8d;
}
.light-theme .main-menu {
background-color: #6653d9;
color: #ffffff;
}
.light-theme .main-menu a:focus,
.light-theme .main-menu a:hover {
background-color: #473c8d;
}
.light-theme .news-view button {
border: 1px solid #6653d9;
color: #6653d9;
}
.light-theme .news-view button:focus,
.light-theme .news-view button:hover,
.light-theme .news-view button:active,
.light-theme .news-view .view-checked {
background-color: #6653d9;
color: #ffffff;
}
.light-theme .news-view .row-view:focus::before,
.light-theme .news-view .row-view:hover::before,
.light-theme .news-view .row-view:active::before {
background-image: url("img/rows-light-checked.svg");
}
.light-theme .news-view .tile-view:focus::before,
.light-theme .news-view .tile-view:hover::before,
.light-theme .news-view .tile-view:active::before {
background-image: url("img/tiles-light-checked.svg");
}
.light-theme .row-view::before {
background-image: url("img/rows-light.svg");
}
.light-theme .tile-view::before {
background-image: url("img/tiles-light.svg");
}
.light-theme .row-view::before {
background-image: url("img/rows-light.svg");
}
.light-theme .row-view.view-checked::before {
background-image: url("img/rows-light-checked.svg");
}
.light-theme .tile-view.view-checked::before {
background-image: url("img/tiles-light-checked.svg");
}
.light-theme .new-block {
background-color: #ffffff;
box-shadow: 0 1px 4px rgba(0, 0, 0, 0.2);
}
.light-theme .new-block time {
color: #aaaaaa;
}
.light-theme .cookies-agreement {
background-color: #fdeacd;
box-shadow: 0 -1px 4px rgba(0, 0, 0, 0.2);
}
.light-theme .button {
background-color: #6653d9;
color: #ffffff;
}
.light-theme .button:focus,
.light-theme .button:hover {
background-color: #473c8d;
}
.light-theme .page-footer {
background-color: #6653d9;
color: #ffffff;
}
.light-theme .subscription {
background-color: #ffffff;
box-shadow: 0 1px 4px rgba(0, 0, 0, 0.2);
}
.light-theme .subscription-message {
background-color: #6653d9;
color: #eae9f2;
}
.light-theme .subscription-message::before {
background-image: url("img/icon-ok-light.svg");
}
.light-theme .subscription-label {
color: #6653d9;
}
.light-theme .subscription-email {
border-bottom: 1px solid #6653d9;
color: #333333;
}
/* Dark theme */
.dark-theme {
color: #f2f2f2;
background-color: #17161a;
}
.dark-theme .page-header {
background-color: #373540;
box-shadow: 0 2px 2px rgba(0, 0, 0, 0.6);
}
.dark-theme .header-link {
color: #9484f2;
}
.dark-theme .header-link::before {
background: url("img/arrow-back-dark.svg") no-repeat 0 0;
}
.dark-theme .theme-button {
color: #9484f2;
border: 1px solid #9484f2;
}
.dark-theme .theme-button::before {
background-image: url("img/sun-normal.svg");
}
.dark-theme .theme-button:focus,
.dark-theme .theme-button:hover {
color: #17161a;
background-color: #9484f2;
}
.dark-theme .theme-button:focus::before,
.dark-theme .theme-button:hover::before {
background-image: url("img/sun-hover.svg");
}
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background-color: #373540;
background-image: url("img/menu-open-dark.svg");
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.dark-theme .menu-open:focus,
.dark-theme .menu-open:hover {
background-color: #473c8d;
background-image: url("img/menu-open-dark.svg");
}
.dark-theme .menu-close {
background-color: #473c8d;
}
.dark-theme .menu-close:focus,
.dark-theme .menu-close:hover {
background-color: #6653d9;
}
.dark-theme .main-menu {
background-color: #473c8d;
color: #f2f2f2;
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.dark-theme .main-menu a:focus,
.dark-theme .main-menu a:hover {
background-color: #6653d9;
}
.dark-theme .news-view button {
border: 1px solid #9484f2;
color: #9484f2;
}
.dark-theme .news-view button:focus,
.dark-theme .news-view button:hover,
.dark-theme .news-view button:active,
.dark-theme .news-view .view-checked {
background-color: #9484f2;
color: #17161a;
}
.dark-theme .news-view .row-view:focus::before,
.dark-theme .news-view .row-view:hover::before,
.dark-theme .news-view .row-view:active::before {
background-image: url("img/rows-dark-checked.svg");
}
.dark-theme .news-view .tile-view:focus::before,
.dark-theme .news-view .tile-view:hover::before,
.dark-theme .news-view .tile-view:active::before {
background-image: url("img/tiles-dark-checked.svg");
}
.dark-theme .row-view::before {
background-image: url("img/rows-dark.svg");
}
.dark-theme .tile-view::before {
background-image: url("img/tiles-dark.svg");
}
.dark-theme .row-view.view-checked::before {
background-image: url("img/rows-dark-checked.svg");
}
.dark-theme .tile-view.view-checked::before {
background-image: url("img/tiles-dark-checked.svg");
}
.dark-theme .new-block {
background-color: #2a2930;
box-shadow: 0 1px 4px rgba(0, 0, 0, 0.6);
}
.dark-theme .new-block time {
color: #888888;
}
.dark-theme .cookies-agreement {
background-color: #473c8d;
box-shadow: 0 -1px 4px rgba(0, 0, 0, 0.6);
}
.dark-theme .button {
background-color: #9484f2;
color: #17161a;
}
.dark-theme .button:focus,
.dark-theme .button:hover {
background-color: #b6aaff;
}
.dark-theme .page-footer {
background-color: #0a0910;
color: #f2f2f2;
}
.dark-theme .subscription {
background-color: #2a2930;
box-shadow: 0 1px 4px rgba(0, 0, 0, 0.6);
}
.dark-theme .subscription-message {
background-color: #9484f2;
color: #17161a;
}
.dark-theme .subscription-message::before {
background-image: url("img/icon-ok-dark.svg");
}
.dark-theme .subscription-label {
color: #9484f2;
}
.dark-theme .subscription-email {
border-bottom: 1px solid #9484f2;
color: #f2f2f2;
}
</div></div></div><div class="course-editor-controls"><button class="course-editor-controls__item course-editor-controls__item--answer" type="button">Show the answer</button></div><div class="js--feedback course-unhappy"><button class="js--feedback-btn course-unhappy__button" type="button"><span class="sr-only">Complain</span><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#unhappy"></use></svg></button><div class="js--feedback-modal course-unhappy__modal course-unhappy__modal--step-one"><form method="POST" action="/courses/intro-to-web-development/javascript/theme-switcher/signal" class="course-unhappy__form form"><input type="hidden" name="csrf_name" value="csrf69aed4b9b39f7"><input type="hidden" name="csrf_value" value="33211908e1e9e30d195997f3258bed7c"><p class="course-unhappy__text">What didn’t you like in this task?</p><div class="course-unhappy__slide"><div class="course-unhappy__wrap"><div class="course-unhappy__list"><label class="checkbox checkbox--small"><input class="checkbox__input" name="problems" value="1" type="checkbox"><span class="checkbox__text">The code check doesn’t work</span></label><label class="checkbox checkbox--small"><input class="checkbox__input" name="problems" value="2" type="checkbox"><span class="checkbox__text">The task is too difficult</span></label><label class="checkbox checkbox--small"><input class="checkbox__input" name="problems" value="3" type="checkbox"><span class="checkbox__text">The theory is hard to understand</span></label><label class="checkbox checkbox--small"><input class="js--feedback-other-checkbox checkbox__input" name="problems" value="0" type="checkbox"><span class="checkbox__text">Other (will write below)</span></label></div><div class="course-unhappy__message"><label class="sr-only" for="id-message">Другое (сейчас напишу)</label><textarea class="field field--area" id="id-message" name="message" placeholder="For example, a typo in the word «heading»"></textarea></div></div></div><button class="course-unhappy__control course-unhappy__control--prev" type="button"><span class="sr-only">Prev</span><svg aria-hidden="true"><use xlink:href="/img/sprites/landings.svg#arrow"></use></svg></button><button class="course-unhappy__control course-unhappy__control--next" type="button" disabled><span class="sr-only">Next</span><svg aria-hidden="true"><use xlink:href="/img/sprites/landings.svg#arrow"></use></svg></button><button class="js--submit-btn button" disabled>Submit</button></form><p class="course-unhappy__result hidden">Thanks! We’ll fix everything at once!</p></div></div></div><div class="course-layout__column course-layout__column--right"><span class="course-separator-control"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-arrows-separator"></use></svg></span><div class="course-browser"><div class="course-browser__menu"><span class="course-browser__menu-item course-browser__address"><span class="course-browser__menu-item-inner"><span class="course-browser__address-text"></span></span></span><button class="js--autorun course-browser__menu-item course-browser__menu-button course-browser__menu-button--text-with-icon course-browser__menu-button--autorun course-browser__menu-button--autorun-on" title="Autorun" type="button"><span class="course-browser__menu-item-inner"><svg aria-hidden="true"><use xlink:href="/img/sprites/courses.svg#icon-autorun"></use></svg>
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