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<!DOCTYPE html>
<html>
<head>
<link id="prefetch" rel="prefetch" href="pass.png">
</head>
<body>
<script>
const bc = new BroadcastChannel(new URLSearchParams(location.search).get("uid"));
document.getElementById("prefetch").addEventListener("error", e => bc.postMessage(false));
const observer = new PerformanceObserver(entries => {
const found = entries.getEntriesByName(new URL("pass.png", location.href).href);
if (found.length)
bc.postMessage(found[0].encodedBodySize > 0);
});
observer.observe({entryTypes: ["resource"]});
</script>
</body>
</html>