> ## Documentation Index
> Fetch the complete documentation index at: https://thrackle.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# ProtocolERC721U Invariants

##### Note: See ERC721-Invariants page one level up which has most up to date information on what has been invarianted for ERC721, nothing has been implemented for the upgradeable ERC721s.

## Protocol Related Invariants

* Version will never be blank
* Version will never change.
* Any user can get the contract's version
* Only app admins may connect a handler
* A non-appAdmin can never connect a handler to the contract
* Any account can retrieve handler address
* Handler address can never be zero address
* New deployment will always emit NewTokenDeployed event

## Base ERC721 Invariants

* Calling balanceOf for the zero address should revert.
* Calling ownerOf for an invalid token should revert.
* approve() should revert on invalid token.
* transferFrom() should revert if caller is not operator.
* transferFrom() should reset approvals.
* transferFrom() should update the token owner.
* transferFrom() should revert if from is the zero address.
* transferFrom() should revert if to is the zero address.
* transferFrom() to self should not break accounting.
* transferFrom() to self should reset approvals.
* safeTransferFrom() should revert if receiver is a contract that does not implement onERC721Received()

## Burnable ERC721 Invariants

* User balance and total supply should be updated correctly when burn is called.
* A token that has been burned cannot be transferred.
* A token that has been burned cannot be transferred.
* A burned token should have its approvals reset.
* getApproved() should revert if the token is burned.
* ownerOf() should revert if the token has been burned.

## Mintable ERC721 Invariants

* User balance and total supply should be updated correctly after minting.

## ProtocolTokenCommon Invariants

* Only an App Admin can propose a new AppManager
* Proposed AppManagerAddress can not be set to zero address
* Any type of address may confirm the proposed AppManager as long as it is the proposed AppManager.
* Only the proposed AppManager may confirm the AppManagerAddress
* When AppManagerAddress is confirmed, AppManagerAddressSet event is always emitted
* Any type of address may retrieve the AppManagerAddress
* Any type of address may retrieve the HandlerAddress

## Upgradeable Invariants

* Contract can only be initialized one time
* When upgrading logc in the contract, the data remains pristine
* When adding a new variable to the end of the storage area and reducing the \_\_gap by 1 array element, the data remains pristine.