pub fn main(world: World) -> Void raises {
    var bytes: [4]u8 = [65, 66, 67, 68]
    var values: [3]i32 = [10, 20, 30]
    let index: usize = 2
    bytes[1] = 90
    values[index] = 42
    if bytes[0] == 65 && bytes[1] == 90 && values[2] == 42 {
        check world.out.write("indexed mutation ok\n")
    }
}