func test_binarySub() {
 // https://www.wolframalpha.com/input?i=-922337203685477587+-+%28-9223372036854775808%29
 let lhs = BigInt("-922337203685477587")!
 let rhs = BigInt("-9223372036854775808")!
 let expected = BigInt("8301034833169298221")!
 XCTAssertEqual(lhs - rhs, expected)
 // The same on Swift.Int:
 XCTAssertEqual(-922337203685477587 - (-9223372036854775808), 8301034833169298221)
}
func test_binarySub_2() {
 typealias Word = BigIntPrototype.Word
 let intMax = Int.max
 let intMaxAsWord = Word(intMax.magnitude)
 // intMax - (-(Word.max - intMaxAsWord)) =
 // intMax - (-Word.max + intMaxAsWord) =
 // intMax + Word.max - intMaxAsWord =
 // Word.max
 let max = BigInt(intMax)
 let value = BigInt(.negative, magnitude: Word.max - intMaxAsWord)
 let expected = BigInt(.positive, magnitude: Word.max)
 XCTAssertEqual(max - value, expected)
}