var initId = 0;
  var world = createWorld();
  var ctx;
  var canvasWidth;
  var canvasHeight;
  var canvasTop;
  var canvasLeft;
  
  function setupWorld(did) {
          if (!did) did = 0;
          world = createWorld();
          initId += did;
          initId %= demos.InitWorlds.length;
          if (initId < 0) initId = demos.InitWorlds.length + initId;
          demos.InitWorlds[initId](world);
  }
  function setupNextWorld() { setupWorld(1); }
  function setupPrevWorld() { setupWorld(-1); }
  function step(cnt) {
          var stepping = false;
          var timeStep = 1.0/60;
          var iteration = 1;
          world.Step(timeStep, iteration);
          ctx.clearRect(0, 0, canvasWidth, canvasHeight);
          drawWorld(world, ctx);
          setTimeout('step(' + (cnt || 0) + ')', 10);
  }
  Event.observe(window, 'load', function() {
          setupWorld();
          ctx = $('canvas').getContext('2d');
          var canvasElm = $('canvas');
          canvasWidth = parseInt(canvasElm.width);
          canvasHeight = parseInt(canvasElm.height);
          canvasTop = parseInt(canvasElm.style.top);
          canvasLeft = parseInt(canvasElm.style.left);
          Event.observe('canvas', 'click', function(e) {
                  //setupNextWorld();
                  if (Math.random() < 0.5) 
                          demos.top.createBall(world, Event.pointerX(e) - canvasLeft, Event.pointerY(e) - canvasTop);
                  else 
                          createBox(world, Event.pointerX(e) - canvasLeft, Event.pointerY(e) - canvasTop, 10, 10, false);
          });
          Event.observe('canvas', 'contextmenu', function(e) {
                  if (e.preventDefault) e.preventDefault();
                  setupPrevWorld();
                  return false;
          });
          step();
  });