Effect.SlideOut = function(element, options) {
  options = $H(options).merge({
    x: (-1 * element.getWidth()),
    mode: 'absolute',
    afterFinish: function(effect) {
      effect.element.hide();
      effect.element.setStyle({left: '0em'});
    }
  });
  new Effect.Move(element, options.toObject());
}
Effect.SlideIn = function(element, options) {
  element.setStyle({left: element.getWidth() + 'px'});
  element.show();
  options = $H(options).merge({
    x: 0,
    mode: 'absolute',
    delay: 0.2
  });
  new Effect.Move(element, options.toObject());
}
Effect.PAIRS['slide_in_out'] = ["SlideIn", "SlideOut"]