def unregister(*args, &block)
      h = Unobservable.handler_for(*args, &block)
      index = @handlers.index(h)
      if index
        @handlers.slice!(index)
        return h
      else
        return nil
      end
    end
def unregister(*args, &block)
      h = Unobservable.handler_for(*args, &block)
      index = @handlers.index(h)
      if index
        @handlers.slice!(index)
        return h
      else
        return nil
      end
    end
Removes a single instance of the specified event handler
 from the list of event handlers.  Therefore, if you've
 registered the same event handler 3 times, then you will
 need to unregister it 3 times as well.