func test_xor_truthTable() {
  typealias Word = BigIntPrototype.Word
  let lhsWord: Word = 0b1100
  let rhsWord: Word = 0b1010

  let lhs = BigInt(.positive, magnitude: lhsWord)
  let rhs = BigInt(.positive, magnitude: rhsWord)

  let expected = BigInt(lhsWord ^ rhsWord)
  XCTAssertEqual(lhs ^ rhs, expected)
  XCTAssertEqual(rhs ^ lhs, expected)
}