pub fn main(world: World) -> Void raises {
    var out: [6]u8 = [0_u8; 6]
    let repeated: Maybe<Span<u8>> = std.str.repeat(out, "ha", 3)
    if repeated.has && std.mem.eql(repeated.value, "hahaha") {
        check world.out.write("repeat string ok\n")
    }
}