Need Help in my Multilevel CSS Accordion Menu

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I am trying to make a CSS multilevel Accordion, But I am having difficulty in creating a Sub Menu inside it. Can anybody tell me how how to add another level in it.
I want it like this.
Menu 1
Menu 1.1
Menu 1.1.1

Menu 2
Menu 2.1
Menu 2.1.1
Menu 2.1.2

Codepen link: http://codepen.io/naeemrind/pen/apyzYZ?editors=1010
JSFiddle: https://jsfiddle.net/naeemrind/53Lhg8xs/

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