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DecimalはReferenceConvertibleではないので、DateとNSDateの関係とは微妙に違いそうですね ただNSDecimalNumber用のenumを使い回してはいるようです https://github.com/apple/swift-corelibs-foundation/blob/ee61823ae71f4c4f9ca157fd290c4b7de915b2a4/Foundation/Decimal.swift
swift-corelibs-foundation - The Foundation Project, providing core utilities, internationalization, and OS independence