Element.toggle & Fx.Slide bug

When i use Element.toggle("slide") to toggle a element to visible or hidden, i found that when the element is visible, and execute toggle("slide") twice in a short time, u will not see that element hidden and shown then, but hidden twice.

in case i can't be understood clearly, i post my test case in attachment, please check it.

and i think if add these codes can resolve this bug:

Fx.Twin.include({
start: function() {

if (this.how == "out" && this.hidden()) {
  return this.cancel();
}

return this.$super();

} });