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chromium/third_party/blink/web_tests/external/wpt/fetch/api/response/multi-globals/incumbent/incumbent.html 

<!DOCTYPE html>
<title>Incumbent page used as a test helper</title>

<iframe src="../current/current.html" id="c"></iframe>
<iframe src="../relevant/relevant.html" id="r"></iframe>

<script>
'use strict';

window.createRedirectResponse = (...args) => {
    const current = document.querySelector('#c').contentWindow;
    const relevant = document.querySelector('#r').contentWindow;
    return current.Response.redirect.call(relevant.Response, ...args);
};

</script>