Coffee shop scenario

;; Background: This scenario is from a self-service ordering touchscreen at a
;; coffee shop that you are assigned to develop. Jill is a customer who wants
;; some coffee.

;; order: Jill starts to order
;; coffee: some coffee at the touchscreen.
;; roast: She adds a small, dark roast.
;; amend-order: Then she adds a
;; coffee: large medium roast
;; complement syrup: with double almond syrup.
;; amend-order: She adds a medium
;; roast: medium roast
;; complement soy milk: with two soy milk shots
;; complement soy milk - second time: (the second one by accident).
;; She reads the price and it's too expensive.
;; amend-order (by index): So she removes the second coffee (the one with the almond syrup).
;; amend-order (remove complement by index to a coffee): Then she realizes the third coffee has two soy milk shots and she removes one.
;; execute-order: The price looks good, so she submits the order.

;; parts
;; - order - [index of coffees]
;; - coffee - [type, complements]
;; - complement - [type]