> ## 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.

# ApplicationERC721 Invariants

## [Base ERC721 Invariants](https://github.com/forte-service-company-ltd/forte-rules-engine/blob/main/test/client/token/ERC721/invariant/ApplicationERC721Base.t.i.sol)

* 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 approved.
* 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.

## [Burnable ERC721 Invariants](https://github.com/forte-service-company-ltd/forte-rules-engine/blob/main/test/client/token/ERC721/invariant/ApplicationERC721MintBurn.t.i.sol)

* The burn function should destroy tokens and reduce the total supply
* A burned token should not be transferrable
* approve() should revert if the token is burned
* getApproved() should revert if the token is burned
* ownerOf() should revert if the token has been burned.
* A burned token should have its approvals reset.

## [Mintable ERC721 Invariants](https://github.com/forte-service-company-ltd/forte-rules-engine/blob/main/test/client/token/ERC721/invariant/ApplicationERC721MintBurn.t.i.sol)

* Mint increases the total supply
* Mint creates a fresh applicationNFT
* Minting tokens should update user's balance

# Not Implemented

## [Protocol Related Invariants](https://github.com/forte-service-company-ltd/forte-rules-engine/blob/main/test/client/token/ERC721/invariant/ApplicationERC721System.t.i.sol)

* 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
* Once the handler address is set to a non zero address, Handler address can never be zero address
* New deployment will always emit NewTokenDeployed event

## Base

* safeTransferFrom() should revert if receiver is a contract that does not implement onERC721Received()

## [ProtocolTokenCommon Invariants](https://github.com/forte-service-company-ltd/forte-rules-engine/blob/main/test/client/token/ERC721/invariant/ApplicationERC721System.t.i.sol)

* 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