Sandbox V1

The function `burnFrom(from, id)` will enable anyone to burn the item, when the item id's operator is set to its owner.

The _burn() function has a 'public' access modifier, which might be invoked by any address.

The check for the validity of the transfer is missing in the function before making modifications to states. In the current code, _checkTransfer() is called before each call of _transferFrom() so the code is safe. However, this pattern is not guaranteed in future implementations

The given input `to` is missing the check for the non-zero address.

In the function balanceOf the return value is missing in the function declaration.

In the function `approveFor`, it can extracted as a common internal function_approveFor() for better code reusability.

According to the package.json, it seems the codebase referenced the library `@openzeppelin/contracts-upgradeable" with a version higher than "^4.0.0". It called us the attention here because openzeppelin released a hotfix for UUPS contract vulnerability for contract version v4.1.0 to v4.3.1. back in September this year. Reference: https://forum.openzeppelin.com/t/security-advisory-initialize-uups-implementation-contracts/15301

the statemnt `408 (size of the land) is hard coded` could be declared as `constant` since these state variables are never to be changed.

The variable x,y,size are using `uint16`, which might cause additional type casting cost.