pub fn main(world: World) -> Void raises {
    var storage: [16]u8 = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    var arena: FixedBufAlloc = std.mem.arena(storage)
    let first: Maybe<MutSpan<u8>> = std.mem.allocBytes(arena, 8)
    if first.has && std.mem.capacity(arena) == 16 {
        std.mem.reset(&mut arena)
        let second: Maybe<MutSpan<u8>> = std.mem.allocBytes(arena, 16)
        if second.has {
            check world.out.write("arena ok\n")
        }
    }
}