<html>
<head>
  <script type="text/javascript">
window.onload = function() {
    var ctx = document.getElementById("c1").getContext("2d");
    ctx.mozFillRule = "evenodd";
    ctx.rect(50, 50, 200, 200);
    ctx.rect(100, 100, 100, 100);
    ctx.clip();

    ctx.beginPath();
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}
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</head>
<body>
  <div><canvas id="c1" width="300" height="300"></canvas></div>
</body>
</html>