def register(*args, &block)
      h = Unobservable.handler_for(*args, &block)
      @handlers << h
      return h
    end
def register(*args, &block)
      h = Unobservable.handler_for(*args, &block)
      @handlers << h
      return h
    end
Registers the given event handler so that it will be
 invoked when the event is raised.