pub fn main(world: World) -> Void raises {
    var buffer: [4]u8 = [0_u8; 4]
    let copy: Maybe<Span<u8>> = std.str.copy(buffer, "zero")
    if copy.has && std.mem.eql(copy.value, "zero") {
        check world.out.write("copy string ok\n")
    }
}