When you're driving a car, do you reach through the dashboard and angle the front wheels slightly to the right? No, you use the control components of the car, namely the steering wheel. It has some sort of relation to the front wheels, but you don't know if that's a mechanical linkage, a power-assisted linkage, or a drive-by-wire arrangement.

In the same way, your A should provide the user interface; any additional modules it uses in the implementation should be hidden from the user.

In real life, things are not always totally hidden, after all, you have to inflate tires, replace a flat tire, check the oil ... but routine operation is through components available to the driver seat.

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