fn isEven(n: usize) -> Bool {
    if n == 0 {
        return true
    }
    let odd: Bool = isOdd(n - 1)
    return odd
}

fn isOdd(n: usize) -> Bool {
    if n == 0 {
        return false
    }
    let even: Bool = isEven(n - 1)
    return even
}

fn checkEven(n: usize) -> Bool {
    let even: Bool = isEven(n)
    return even
}

fn checkOdd(n: usize) -> Bool {
    let odd: Bool = isOdd(n)
    return odd
}

pub fn main(world: World) -> Void raises {
    if checkEven(8) && checkOdd(7) && !isEven(3) {
        check world.out.write("mutual recursion ok\n")
    }
}