t.Errorf("ContainsFunc(%q, func(%q)) = %v, want %v",
				ct.b, ct.r, !ct.expected, ct.expected)
		}
	}
}

var makeFieldsInput = func() []byte {
	x := make([]byte, 1<<20)
	// Input is ~10% space, ~10% 2-byte UTF-8, rest ASCII non-space.
	r := rand.New(rand.NewSource(99))
	for i := range x {
		switch r.Intn(10) {
		case 0:
			x[i] = ' '
		case 1:
			if i > 0 && x[i-1] == 'x' {
				copy(x[i-1:], "χ")