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<defs>
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        <rect width="10" height="10" fill="red" stroke="none"/>
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    <marker id="markerMiddle" viewBox="0 0 10 10" markerWidth="2" markerHeight="2" refX="5" refY="5" markerUnits="strokeWidth">
        <circle cx="5" cy="5" r="5" fill="green" stroke="none"/>
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    <marker id="markerEnd" viewBox="0 0 10 10" markerWidth="2" markerHeight="2" refX="5" refY="5" markerUnits="strokeWidth">
        <path d="M 5 0 L 10 10 L 0 10 Z" fill="blue" stroke="none"/>
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<g onclick="mouseHandler()">
    <path fill="none" stroke="black" stroke-width="8" marker-start="url(#markerStart)" marker-mid="url(#markerMiddle)" marker-end="url(#markerEnd)" d="M 130 135 L 180 135 L 180 185"/>
</g>

<script>
function mouseHandler(evt)
{
    var marker = document.getElementById("markerStart");
    marker.setAttribute("markerWidth", "2");
}

if (window.eventSender) {
    eventSender.mouseMoveTo(150, 135);
    eventSender.mouseDown();
    eventSender.mouseUp();
}
</script>

</svg>