rr := make(nat, numWords)
		copy(rr, x)
		x = rr
	}

	// Ideally the precomputations would be performed outside, and reused
	// k0 = -m**-1 mod 2**_W. Algorithm from: Dumas, J.G. "On Newton–Raphson
	// Iteration for Multiplicative Inverses Modulo Prime Powers".
	k0 := 2 - m[0]
	t := m[0] - 1
	for i := 1; i < _W; i <<= 1 {
		t *= t
		k0 *= (t + 1)
	}
	k0 = -k0

		"einstein":   {Data: []byte("Albert\n")},
		"dir/newton": {Data: []byte("Sir Isaac\n")},
	}
	tmpDir = t.TempDir()
	if err := CopyFS(tmpDir, fsys); err != nil {
		t.Fatal("CopyFS:", err)
	}
	forceMFTUpdateOnWindows(t, tmpDir)
	tmpFsys = DirFS(tmpDir)
	if err := fstest.TestFS(tmpFsys, "william", "carl", "daVinci", "einstein", "dir/newton"); err != nil {
		t.Fatal("TestFS:", err)
	}