The core abstraction is that in any case, currencies are the North Star. If no monetary units are involved it means that the exchange happened directly. If monetary units are involved, it just means that the exact same exchange happened, but indirectly.
A direct exchange means a crisscrossing of hearts? Is that what you meant?
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A direct exchange is a criss-crossing of anything that doesn't involve a monetary unit, be it material or immaterial. An indirect exchange involves a monetary unit, for the same final effect as the direct exchange.
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