I'd point you to www.bibocurrency.com and mention that the function of measurement is invalidated by the function of storing value in the same way that the function of a unit of measure of length is invalidated by making a specific length of rope as the only unit of measure. The site though does a much more thorough job at demonstrating how one function invalidates the other functions of currency and details exactly why money is, and what is only tokens or stores of value, as money and currency don't have any difference since currency by it's very tangible premise of your definition with "portable, durable" is describing a store of value.

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