  }
  
  func testThreeElementSequence() {
    let pairs = (0...).prefix(3).adjacentPairs()
    XCTAssertEqualSequences(pairs, [(0, 1), (1, 2)], by: ==)
  }
  
  func testManySequences() {
    for n in 4...100 {
      let pairs = (0...).prefix(n).adjacentPairs()
      XCTAssertEqualSequences(pairs, zip(0..., 1...).prefix(n - 1), by: ==)
    }
  }
  