func isValidEmail(email string) bool {
        // Basic email validation using regular expression
        emailRegex := regexp.MustCompile(`^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$`)
        return emailRegex.MatchString(email)
}

func cleanUserList(users []User) []User {
        return WithoutBy(users, func(u User) bool { return !isValidEmail(u.Email) })
}

func isEven(num int) bool {
        return num%2 == 0
}

func removeEven(numbers []int) []int {
        return WithoutBy(numbers, isEven)
}