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https://qiita.com/yyu/items/e8f1b4a17dac4f9108a5 public protocol HList { } public struct HNil: HList { init() { } } public struct HCons<H, T: HList>: HList { public let head: H public let tail: T public init(_ h: H, _ t: T) { self.head = h self.tail = t } }