code: public static WipeAnimation unreveal (SimplePanel target)
      {
          return new WipeAnimation(target) {
              @Override protected int computeCurHeight (int targetHeight, double progress) {
                  return (int) ((1-progress) * targetHeight);
              }
          };
      }
docs: Creates an animation that unreveals the supplied target panel (wiping it down to height
  zero). The animation can be subsequently configured via its fluent methods. The returned
  animation is configured fluently, for example:
  
  <code>FX.unreveal(target).fromBottom().onComplete(action).run(250);</code>