def value=(val)
      if @options.include? val
        old = @value
        @value = @buttons[0].text = val
        @on_changed.call(old, val) if @on_changed
      end
    end
def value=(val)
      if @options.include? val
        old = @value
        @value = @buttons[0].text = val
        @on_changed.call(old, val) if @on_changed
      end
    end
Sets the currently selected value of the drop-down list. It is ignored if
 it is not among the available options.