How can I remove the duplicated Then clauses in these specs?

event_bus_spec.rb

context 'EventBus methods cascade' do

  context 'clear' do
    When(:result) { EventBus.clear }
    Then { result.should == EventBus }
  end

  context 'publish' do
    When(:result) { EventBus.publish('aa123bb', {}) }
    Then { result.should == EventBus }
  end

  context 'subscribe' do
    When(:result) { EventBus.subscribe('aa123bb', listener, :handler) }
    Then { result.should == EventBus }
  end

end

event_bus_spec2.rb

context 'when called incorrectly' do

  context 'when specifying the event name' do

    context 'must provide a method or a block' do
      When(:subscribe) { EventBus.subscribe('blah', listener) }
      Then { subscribe.should have_failed(ArgumentError) }
    end

    context 'cannot provide a method AND a block' do
      When(:subscribe) { EventBus.subscribe('blah', listener, :handler) {|info| }}
      Then { subscribe.should have_failed(ArgumentError) }
    end

    context 'must provide a block when no method is supplied' do
      When(:subscribe) { EventBus.subscribe('blah') }
      Then { subscribe.should have_failed(ArgumentError) }
    end

  end

  context 'when specifying a listener object' do

    context 'when a method is also provided' do
      When(:subscribe) { EventBus.subscribe(listener, double) }
      Then { subscribe.should have_failed(ArgumentError) }
    end

    context 'when a block is also provided' do
      When(:subscribe) { EventBus.subscribe(listener) {|info| } }
      Then { subscribe.should have_failed(ArgumentError) }
    end

  end

end