Кнопка включения / выключения анимации

<svg xmlns="http://www.w3.org/2000/svg" onload="init()" width="320" height="240" encoding="utf-8">
  <script type="text/ecmascript">
    <![CDATA[
 
      function init()
      {
        window.rot = 0;
        window.vent = document.getElementById( "vent");
        window.timer = null;
      }
 
      function startAnimation() {
        if(timer) return;
        motion();
        };
        
      function stopAnimation()
      {
        clearTimeout(timer);
        timer = null;
      } ;
 
      function motion() {
        rot += 5;
        vent.setAttributeNS( null, "transform", "translate(160, 120), rotate(" + rot + ")");
        timer = setTimeout( "motion()", 30);
        } ;
    ]]>
  </script>
 
  <title>Rotation</title>
  <g id="body">
<g>
<line x1="160" y1="120" x2="160" y2="200" stroke="black" stroke-width="10"/>
<line x1="130" y1="200" x2="190" y2="200" stroke="black" stroke-width="10"/>
</g>
    <g id="vent" stroke="red" transform="translate(160, 120)">
      <line x1="-30" y1="-30" x2="30" y2="30" stroke-width="10" />
      <line x1="-30" y1="30" x2="30" y2="-30" stroke-width="10" />
      <circle cx="0" cy="0" r="6" fill="silver" stroke="black" stroke-width="1" />
    </g>
  </g>
 
</svg>
  <button onclick="stopAnimation();"> Stop</button>
  <button onclick="startAnimation();"> Start</button>