Add at least some comments

Welt.java

@@ -35,7 +35,11 @@ class Welt implements Drawable
         ArrayList<Rectangle> remove = new ArrayList<Rectangle>();
         for (Rectangle r : _obstacles)
         {
+            // Move all obstacles accoring to speed
             r.setPos(r.getX() - (elapsed * 1e-3 * _speed), r.getY());
+
+            // Mark obstacles that are off screen for removal to avoid
+            // concurrent modifications
             if (r.getX() < -r.getWidth())
             {
                 remove.add(r);
@@ -45,11 +49,15 @@ class Welt implements Drawable
         {
             _obstacles.remove(r);
         }
+
+        // Add a new obstacle after DISTANCE
         Rectangle lastObstacle = _obstacles.get(_obstacles.size() - 1);
         if (lastObstacle.getX() + lastObstacle.getWidth() < TurtleWelt.WIDTH - DISTANCE)
         {
             createObstacle(TurtleWelt.WIDTH);
         }
+
+        // Increase speed according to acceleration
         _speed += _acc * elapsed * 1e-3;
     }