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https://github.com/apple/swift-numerics/blob/master/Sources/Complex/Arithmetic.swift ComplexSignedNumericじゃないんですね。 https://developer.apple.com/documentation/swift/signednumeric
The SignedNumeric protocol extends the operations defined by the Numeric protocol to include a value’s additive inverse.
定義的には適合しそうですがどうなんでしょう。
Numerical APIs for Swift. Contribute to apple/swift-numerics development by creating an account on GitHub.