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openzeppelin-contracts/certora/specs/ERC721.spec

679 lines
38 KiB

import "helpers/helpers.spec";
import "methods/IERC721.spec";
import "methods/IERC721Receiver.spec";
methods {
// exposed for FV
function mint(address,uint256) external;
function safeMint(address,uint256) external;
function safeMint(address,uint256,bytes) external;
function burn(uint256) external;
function unsafeOwnerOf(uint256) external returns (address) envfree;
function unsafeGetApproved(uint256) external returns (address) envfree;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Helpers │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
definition authSanity(env e) returns bool = e.msg.sender != 0;
// Could be broken in theory, but not in practice
definition balanceLimited(address account) returns bool = balanceOf(account) < max_uint256;
function helperTransferWithRevert(env e, method f, address from, address to, uint256 tokenId) {
if (f.selector == sig:transferFrom(address,address,uint256).selector) {
transferFrom@withrevert(e, from, to, tokenId);
} else if (f.selector == sig:safeTransferFrom(address,address,uint256).selector) {
safeTransferFrom@withrevert(e, from, to, tokenId);
} else if (f.selector == sig:safeTransferFrom(address,address,uint256,bytes).selector) {
bytes params;
require params.length < 0xffff;
safeTransferFrom@withrevert(e, from, to, tokenId, params);
} else {
calldataarg args;
f@withrevert(e, args);
}
}
function helperMintWithRevert(env e, method f, address to, uint256 tokenId) {
if (f.selector == sig:mint(address,uint256).selector) {
mint@withrevert(e, to, tokenId);
} else if (f.selector == sig:safeMint(address,uint256).selector) {
safeMint@withrevert(e, to, tokenId);
} else if (f.selector == sig:safeMint(address,uint256,bytes).selector) {
bytes params;
require params.length < 0xffff;
safeMint@withrevert(e, to, tokenId, params);
} else {
require false;
}
}
function helperSoundFnCall(env e, method f) {
if (f.selector == sig:mint(address,uint256).selector) {
address to; uint256 tokenId;
require balanceLimited(to);
requireInvariant notMintedUnset(tokenId);
mint(e, to, tokenId);
} else if (f.selector == sig:safeMint(address,uint256).selector) {
address to; uint256 tokenId;
require balanceLimited(to);
requireInvariant notMintedUnset(tokenId);
safeMint(e, to, tokenId);
} else if (f.selector == sig:safeMint(address,uint256,bytes).selector) {
address to; uint256 tokenId; bytes data;
require data.length < 0xffff;
require balanceLimited(to);
requireInvariant notMintedUnset(tokenId);
safeMint(e, to, tokenId, data);
} else if (f.selector == sig:burn(uint256).selector) {
uint256 tokenId;
requireInvariant ownerHasBalance(tokenId);
requireInvariant notMintedUnset(tokenId);
burn(e, tokenId);
} else if (f.selector == sig:transferFrom(address,address,uint256).selector) {
address from; address to; uint256 tokenId;
require balanceLimited(to);
requireInvariant ownerHasBalance(tokenId);
requireInvariant notMintedUnset(tokenId);
transferFrom(e, from, to, tokenId);
} else if (f.selector == sig:safeTransferFrom(address,address,uint256).selector) {
address from; address to; uint256 tokenId;
require balanceLimited(to);
requireInvariant ownerHasBalance(tokenId);
requireInvariant notMintedUnset(tokenId);
safeTransferFrom(e, from, to, tokenId);
} else if (f.selector == sig:safeTransferFrom(address,address,uint256,bytes).selector) {
address from; address to; uint256 tokenId; bytes data;
require data.length < 0xffff;
require balanceLimited(to);
requireInvariant ownerHasBalance(tokenId);
requireInvariant notMintedUnset(tokenId);
safeTransferFrom(e, from, to, tokenId, data);
} else {
calldataarg args;
f(e, args);
}
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Ghost & hooks: ownership count │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
ghost mathint _ownedTotal {
init_state axiom _ownedTotal == 0;
}
ghost mapping(address => mathint) _ownedByUser {
init_state axiom forall address a. _ownedByUser[a] == 0;
}
hook Sstore _owners[KEY uint256 tokenId] address newOwner (address oldOwner) STORAGE {
_ownedByUser[newOwner] = _ownedByUser[newOwner] + to_mathint(newOwner != 0 ? 1 : 0);
_ownedByUser[oldOwner] = _ownedByUser[oldOwner] - to_mathint(oldOwner != 0 ? 1 : 0);
_ownedTotal = _ownedTotal + to_mathint(newOwner != 0 ? 1 : 0) - to_mathint(oldOwner != 0 ? 1 : 0);
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Ghost & hooks: sum of all balances │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
ghost mathint _supply {
init_state axiom _supply == 0;
}
ghost mapping(address => mathint) _balances {
init_state axiom forall address a. _balances[a] == 0;
}
hook Sstore _balances[KEY address addr] uint256 newValue (uint256 oldValue) STORAGE {
_supply = _supply - oldValue + newValue;
}
// TODO: This used to not be necessary. We should try to remove it. In order to do so, we will probably need to add
// many "preserved" directive that require the "balanceOfConsistency" invariant on the accounts involved.
hook Sload uint256 value _balances[KEY address user] STORAGE {
require _balances[user] == to_mathint(value);
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Invariant: number of owned tokens is the sum of all balances │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
invariant ownedTotalIsSumOfBalances()
_ownedTotal == _supply
{
preserved mint(address to, uint256 tokenId) with (env e) {
require balanceLimited(to);
}
preserved safeMint(address to, uint256 tokenId) with (env e) {
require balanceLimited(to);
}
preserved safeMint(address to, uint256 tokenId, bytes data) with (env e) {
require balanceLimited(to);
}
preserved burn(uint256 tokenId) with (env e) {
requireInvariant ownerHasBalance(tokenId);
requireInvariant balanceOfConsistency(ownerOf(tokenId));
}
preserved transferFrom(address from, address to, uint256 tokenId) with (env e) {
require balanceLimited(to);
requireInvariant ownerHasBalance(tokenId);
requireInvariant balanceOfConsistency(from);
requireInvariant balanceOfConsistency(to);
}
preserved safeTransferFrom(address from, address to, uint256 tokenId) with (env e) {
require balanceLimited(to);
requireInvariant ownerHasBalance(tokenId);
requireInvariant balanceOfConsistency(from);
requireInvariant balanceOfConsistency(to);
}
preserved safeTransferFrom(address from, address to, uint256 tokenId, bytes data) with (env e) {
require balanceLimited(to);
requireInvariant ownerHasBalance(tokenId);
requireInvariant balanceOfConsistency(from);
requireInvariant balanceOfConsistency(to);
}
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Invariant: balanceOf is the number of tokens owned │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
invariant balanceOfConsistency(address user)
to_mathint(balanceOf(user)) == _ownedByUser[user] &&
to_mathint(balanceOf(user)) == _balances[user]
{
preserved {
require balanceLimited(user);
}
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Invariant: owner of a token must have some balance │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
invariant ownerHasBalance(uint256 tokenId)
balanceOf(ownerOf(tokenId)) > 0
{
preserved {
requireInvariant balanceOfConsistency(ownerOf(tokenId));
require balanceLimited(ownerOf(tokenId));
}
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: balance of address(0) is 0 │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule zeroAddressBalanceRevert() {
balanceOf@withrevert(0);
assert lastReverted;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Invariant: address(0) has no authorized operator │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
invariant zeroAddressHasNoApprovedOperator(address a)
!isApprovedForAll(0, a)
{
preserved with (env e) {
require nonzerosender(e);
}
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Invariant: tokens that do not exist are not owned and not approved │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
invariant notMintedUnset(uint256 tokenId)
unsafeOwnerOf(tokenId) == 0 => unsafeGetApproved(tokenId) == 0;
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: unsafeOwnerOf and unsafeGetApproved don't revert + ownerOf and getApproved revert if token does not exist │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule notMintedRevert(uint256 tokenId) {
requireInvariant notMintedUnset(tokenId);
address _owner = unsafeOwnerOf@withrevert(tokenId);
assert !lastReverted;
address _approved = unsafeGetApproved@withrevert(tokenId);
assert !lastReverted;
address owner = ownerOf@withrevert(tokenId);
assert lastReverted <=> _owner == 0;
assert !lastReverted => _owner == owner;
address approved = getApproved@withrevert(tokenId);
assert lastReverted <=> _owner == 0;
assert !lastReverted => _approved == approved;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rules: total supply can only change through mint and burn │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule supplyChange(env e) {
require nonzerosender(e);
requireInvariant zeroAddressHasNoApprovedOperator(e.msg.sender);
mathint supplyBefore = _supply;
method f; helperSoundFnCall(e, f);
mathint supplyAfter = _supply;
assert supplyAfter > supplyBefore => (
supplyAfter == supplyBefore + 1 &&
(
f.selector == sig:mint(address,uint256).selector ||
f.selector == sig:safeMint(address,uint256).selector ||
f.selector == sig:safeMint(address,uint256,bytes).selector
)
);
assert supplyAfter < supplyBefore => (
supplyAfter == supplyBefore - 1 &&
f.selector == sig:burn(uint256).selector
);
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rules: balanceOf can only change through mint, burn or transfers. balanceOf cannot change by more than 1. │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule balanceChange(env e, address account) {
requireInvariant balanceOfConsistency(account);
require balanceLimited(account);
mathint balanceBefore = balanceOf(account);
method f; helperSoundFnCall(e, f);
mathint balanceAfter = balanceOf(account);
// balance can change by at most 1
assert balanceBefore != balanceAfter => (
balanceAfter == balanceBefore - 1 ||
balanceAfter == balanceBefore + 1
);
// only selected function can change balances
assert balanceBefore != balanceAfter => (
f.selector == sig:transferFrom(address,address,uint256).selector ||
f.selector == sig:safeTransferFrom(address,address,uint256).selector ||
f.selector == sig:safeTransferFrom(address,address,uint256,bytes).selector ||
f.selector == sig:mint(address,uint256).selector ||
f.selector == sig:safeMint(address,uint256).selector ||
f.selector == sig:safeMint(address,uint256,bytes).selector ||
f.selector == sig:burn(uint256).selector
);
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rules: ownership can only change through mint, burn or transfers. │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule ownershipChange(env e, uint256 tokenId) {
require nonzerosender(e);
requireInvariant zeroAddressHasNoApprovedOperator(e.msg.sender);
address ownerBefore = unsafeOwnerOf(tokenId);
method f; helperSoundFnCall(e, f);
address ownerAfter = unsafeOwnerOf(tokenId);
assert ownerBefore == 0 && ownerAfter != 0 => (
f.selector == sig:mint(address,uint256).selector ||
f.selector == sig:safeMint(address,uint256).selector ||
f.selector == sig:safeMint(address,uint256,bytes).selector
);
assert ownerBefore != 0 && ownerAfter == 0 => (
f.selector == sig:burn(uint256).selector
);
assert (ownerBefore != ownerAfter && ownerBefore != 0 && ownerAfter != 0) => (
f.selector == sig:transferFrom(address,address,uint256).selector ||
f.selector == sig:safeTransferFrom(address,address,uint256).selector ||
f.selector == sig:safeTransferFrom(address,address,uint256,bytes).selector
);
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rules: token approval can only change through approve or transfers (implicitly). │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule approvalChange(env e, uint256 tokenId) {
address approvalBefore = unsafeGetApproved(tokenId);
method f; helperSoundFnCall(e, f);
address approvalAfter = unsafeGetApproved(tokenId);
// approve can set any value, other functions reset
assert approvalBefore != approvalAfter => (
f.selector == sig:approve(address,uint256).selector ||
(
(
f.selector == sig:transferFrom(address,address,uint256).selector ||
f.selector == sig:safeTransferFrom(address,address,uint256).selector ||
f.selector == sig:safeTransferFrom(address,address,uint256,bytes).selector ||
f.selector == sig:burn(uint256).selector
) && approvalAfter == 0
)
);
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rules: approval for all tokens can only change through isApprovedForAll. │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule approvedForAllChange(env e, address owner, address spender) {
bool approvedForAllBefore = isApprovedForAll(owner, spender);
method f; helperSoundFnCall(e, f);
bool approvedForAllAfter = isApprovedForAll(owner, spender);
assert approvedForAllBefore != approvedForAllAfter => f.selector == sig:setApprovalForAll(address,bool).selector;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: transferFrom behavior and side effects │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule transferFrom(env e, address from, address to, uint256 tokenId) {
require nonpayable(e);
require authSanity(e);
address operator = e.msg.sender;
uint256 otherTokenId;
address otherAccount;
requireInvariant ownerHasBalance(tokenId);
require balanceLimited(to);
uint256 balanceOfFromBefore = balanceOf(from);
uint256 balanceOfToBefore = balanceOf(to);
uint256 balanceOfOtherBefore = balanceOf(otherAccount);
address ownerBefore = unsafeOwnerOf(tokenId);
address otherOwnerBefore = unsafeOwnerOf(otherTokenId);
address approvalBefore = unsafeGetApproved(tokenId);
address otherApprovalBefore = unsafeGetApproved(otherTokenId);
transferFrom@withrevert(e, from, to, tokenId);
bool success = !lastReverted;
// liveness
assert success <=> (
from == ownerBefore &&
from != 0 &&
to != 0 &&
(operator == from || operator == approvalBefore || isApprovedForAll(ownerBefore, operator))
);
// effect
assert success => (
to_mathint(balanceOf(from)) == balanceOfFromBefore - assert_uint256(from != to ? 1 : 0) &&
to_mathint(balanceOf(to)) == balanceOfToBefore + assert_uint256(from != to ? 1 : 0) &&
unsafeOwnerOf(tokenId) == to &&
unsafeGetApproved(tokenId) == 0
);
// no side effect
assert balanceOf(otherAccount) != balanceOfOtherBefore => (otherAccount == from || otherAccount == to);
assert unsafeOwnerOf(otherTokenId) != otherOwnerBefore => otherTokenId == tokenId;
assert unsafeGetApproved(otherTokenId) != otherApprovalBefore => otherTokenId == tokenId;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: safeTransferFrom behavior and side effects │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule safeTransferFrom(env e, method f, address from, address to, uint256 tokenId) filtered { f ->
f.selector == sig:safeTransferFrom(address,address,uint256).selector ||
f.selector == sig:safeTransferFrom(address,address,uint256,bytes).selector
} {
require nonpayable(e);
require authSanity(e);
address operator = e.msg.sender;
uint256 otherTokenId;
address otherAccount;
requireInvariant ownerHasBalance(tokenId);
require balanceLimited(to);
uint256 balanceOfFromBefore = balanceOf(from);
uint256 balanceOfToBefore = balanceOf(to);
uint256 balanceOfOtherBefore = balanceOf(otherAccount);
address ownerBefore = unsafeOwnerOf(tokenId);
address otherOwnerBefore = unsafeOwnerOf(otherTokenId);
address approvalBefore = unsafeGetApproved(tokenId);
address otherApprovalBefore = unsafeGetApproved(otherTokenId);
helperTransferWithRevert(e, f, from, to, tokenId);
bool success = !lastReverted;
assert success <=> (
from == ownerBefore &&
from != 0 &&
to != 0 &&
(operator == from || operator == approvalBefore || isApprovedForAll(ownerBefore, operator))
);
// effect
assert success => (
to_mathint(balanceOf(from)) == balanceOfFromBefore - assert_uint256(from != to ? 1: 0) &&
to_mathint(balanceOf(to)) == balanceOfToBefore + assert_uint256(from != to ? 1: 0) &&
unsafeOwnerOf(tokenId) == to &&
unsafeGetApproved(tokenId) == 0
);
// no side effect
assert balanceOf(otherAccount) != balanceOfOtherBefore => (otherAccount == from || otherAccount == to);
assert unsafeOwnerOf(otherTokenId) != otherOwnerBefore => otherTokenId == tokenId;
assert unsafeGetApproved(otherTokenId) != otherApprovalBefore => otherTokenId == tokenId;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: mint behavior and side effects │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule mint(env e, address to, uint256 tokenId) {
require nonpayable(e);
requireInvariant notMintedUnset(tokenId);
uint256 otherTokenId;
address otherAccount;
require balanceLimited(to);
mathint supplyBefore = _supply;
uint256 balanceOfToBefore = balanceOf(to);
uint256 balanceOfOtherBefore = balanceOf(otherAccount);
address ownerBefore = unsafeOwnerOf(tokenId);
address otherOwnerBefore = unsafeOwnerOf(otherTokenId);
mint@withrevert(e, to, tokenId);
bool success = !lastReverted;
// liveness
assert success <=> (
ownerBefore == 0 &&
to != 0
);
// effect
assert success => (
_supply == supplyBefore + 1 &&
to_mathint(balanceOf(to)) == balanceOfToBefore + 1 &&
unsafeOwnerOf(tokenId) == to
);
// no side effect
assert balanceOf(otherAccount) != balanceOfOtherBefore => otherAccount == to;
assert unsafeOwnerOf(otherTokenId) != otherOwnerBefore => otherTokenId == tokenId;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: safeMint behavior and side effects │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule safeMint(env e, method f, address to, uint256 tokenId) filtered { f ->
f.selector == sig:safeMint(address,uint256).selector ||
f.selector == sig:safeMint(address,uint256,bytes).selector
} {
require nonpayable(e);
requireInvariant notMintedUnset(tokenId);
uint256 otherTokenId;
address otherAccount;
require balanceLimited(to);
mathint supplyBefore = _supply;
uint256 balanceOfToBefore = balanceOf(to);
uint256 balanceOfOtherBefore = balanceOf(otherAccount);
address ownerBefore = unsafeOwnerOf(tokenId);
address otherOwnerBefore = unsafeOwnerOf(otherTokenId);
helperMintWithRevert(e, f, to, tokenId);
bool success = !lastReverted;
assert success <=> (
ownerBefore == 0 &&
to != 0
);
// effect
assert success => (
_supply == supplyBefore + 1 &&
to_mathint(balanceOf(to)) == balanceOfToBefore + 1 &&
unsafeOwnerOf(tokenId) == to
);
// no side effect
assert balanceOf(otherAccount) != balanceOfOtherBefore => otherAccount == to;
assert unsafeOwnerOf(otherTokenId) != otherOwnerBefore => otherTokenId == tokenId;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: burn behavior and side effects │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule burn(env e, uint256 tokenId) {
require nonpayable(e);
address from = unsafeOwnerOf(tokenId);
uint256 otherTokenId;
address otherAccount;
requireInvariant ownerHasBalance(tokenId);
mathint supplyBefore = _supply;
uint256 balanceOfFromBefore = balanceOf(from);
uint256 balanceOfOtherBefore = balanceOf(otherAccount);
address ownerBefore = unsafeOwnerOf(tokenId);
address otherOwnerBefore = unsafeOwnerOf(otherTokenId);
address otherApprovalBefore = unsafeGetApproved(otherTokenId);
burn@withrevert(e, tokenId);
bool success = !lastReverted;
// liveness
assert success <=> (
ownerBefore != 0
);
// effect
assert success => (
_supply == supplyBefore - 1 &&
to_mathint(balanceOf(from)) == balanceOfFromBefore - 1 &&
unsafeOwnerOf(tokenId) == 0 &&
unsafeGetApproved(tokenId) == 0
);
// no side effect
assert balanceOf(otherAccount) != balanceOfOtherBefore => otherAccount == from;
assert unsafeOwnerOf(otherTokenId) != otherOwnerBefore => otherTokenId == tokenId;
assert unsafeGetApproved(otherTokenId) != otherApprovalBefore => otherTokenId == tokenId;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: approve behavior and side effects │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule approve(env e, address spender, uint256 tokenId) {
require nonpayable(e);
require authSanity(e);
address caller = e.msg.sender;
address owner = unsafeOwnerOf(tokenId);
uint256 otherTokenId;
address otherApprovalBefore = unsafeGetApproved(otherTokenId);
approve@withrevert(e, spender, tokenId);
bool success = !lastReverted;
// liveness
assert success <=> (
owner != 0 &&
(owner == caller || isApprovedForAll(owner, caller))
);
// effect
assert success => unsafeGetApproved(tokenId) == spender;
// no side effect
assert unsafeGetApproved(otherTokenId) != otherApprovalBefore => otherTokenId == tokenId;
}
/*
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ Rule: setApprovalForAll behavior and side effects │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
*/
rule setApprovalForAll(env e, address operator, bool approved) {
require nonpayable(e);
address owner = e.msg.sender;
address otherOwner;
address otherOperator;
bool otherIsApprovedForAllBefore = isApprovedForAll(otherOwner, otherOperator);
setApprovalForAll@withrevert(e, operator, approved);
bool success = !lastReverted;
// liveness
assert success <=> operator != 0;
// effect
assert success => isApprovedForAll(owner, operator) == approved;
// no side effect
assert isApprovedForAll(otherOwner, otherOperator) != otherIsApprovedForAllBefore => (
otherOwner == owner &&
otherOperator == operator
);
}