fn main() {
    println!("Hello, world! {}", factorial(8));
}

fn factorial<T: num::One + std::ops::Mul + std::ops::Sub<Output = T> + PartialEq + Copy>(
    a: T,
) -> T {
    if a == T::one() {
        a
    } else {
        a * factorial(a - T::one())
    }
}