cic-contracts/python/README.md

# Smart contract interfaces

## Technology

CIC smart contracts are implemented using the *solidity* programming
language for the (Ethereum Virtual Machine (EVM).

## Adopted standards

### Signing

### ERC - Direct use

The following well-known solidity interfaces are used directly.

- [ERC20 - Token Standard](https://eips.ethereum.org/EIPS/eip-20)

- [ERC165 - Standard Interface
  Detection](https://eips.ethereum.org/EIPS/eip-165)

- [ERC173 - Contract Ownership
  Standard](https://eips.ethereum.org/EIPS/eip-173)

- [ERC191 - Signed Data
  Standard](https://eips.ethereum.org/EIPS/eip-191)

- [ERC712 - Typed structured data hashing and
  signing](https://eips.ethereum.org/EIPS/eip-712)

- [ERC721 - Non-Fungible Token
  Standard](https://eips.ethereum.org/EIPS/eip-721)

- [ERC5007 - Time NFT (EIP-721 Time
  Extension)](https://eips.ethereum.org/EIPS/eip-5007)

- [ERC5192 - Minimal Soulbound
  NFTs](https://eips.ethereum.org/EIPS/eip-5192)

### ERCs Partial use

The following well-known solidity interfaces are partially implemented
in CIC native interfaces.

- [ERC5679 - Token Minting and
  Burning](https://eips.ethereum.org/EIPS/eip-5679) (See `Minter`,
  `Burner`)

## Native interfaces

### Accounts Index

Append-only list of addresses. Typically used for access control lists.

Addresses may be *added*, *removed*, aswell as *deactivated* and
*activated*. Deactivated accounts still count towards the `entryCount`.

The `entry` method is used to iterate the account list. The order of
which accounts are returned is not guaranteed. Any returned value
matching `address(0x00)` should be skipped, and not counted towards
`entryCount`.

May optionally record time when account was added.

#### ERC165 Interface identifier

b7bca625

#### Solidity interface definition

    interface IAccountsIndex {
        // Address added to store, index in array.
        event AddressAdded(uint256 indexed _idx, address _account);

        // Return number of entries in index.
        function entryCount() external view returns (uint256);

        // Return entry at the spceificed index.
        // Will revert if index is beyond array length.
        // An entry result of 0 means the entry should be skipped, and not count towards entry count.
        function entry(uint256) external view returns (address);

        // Add an entry to the index. Incresases the entry count.
        function add(address) external returns (bool);

        // Verify that the entry exists in the index.
        function have(address) external view returns (bool);

        // Retrieve the timestamp when account was added.
        // If time is not being tracked, a value of 0 should be returned.
        function time(address) external view returns (uint256);
    }

#### Reference implementation

<git://holbrook.no/eth-accounts-index.git> (v0.5.1)

### Accounts Index Mutable

Extends the functionality of `Accounts Index` to allow changes to the
address list.

Addresses may be *added*, *removed*, aswell as *deactivated* and
*activated*. Deactivated accounts still count towards the `entryCount`.

#### ERC165 Interface identifier

9479f0ae

#### Solidity interface definition

    interface IAccountsIndexMutable {
        // Active status of address changed, and by whom changed.
        event AddressActive(address indexed _account, bool _active);

        // Address removed from store, and by whom removed.
        event AddressRemoved(address _account);

        // Remove an entry from the index. Reduces the entry count.
        function remove(address) external returns (bool);

        // Deactivate account but keep in index. Does not affect entry count.
        function deactivate(address) external returns (bool);

        // Activate previously deactivated account. Does not affect entry count.
        function activate(address) external returns (bool);

        // Check if account exists and is active;
        function isActive(address) external view returns (bool);
    }

#### Reference implementation

<git://holbrook.no/eth-accounts-index.git> (v0.5.1)

### Burner

Attached to `ERC20` and `ERC721` tokens that may be *burned*.

Implements the `burn(...)` part of `ERC5679` for interoperability.

#### ERC165 Interface identifier

b1110c1b

#### Solidity interface definition

    interface IBurner {
        // Token(s) successfully burned; by who and how much.
        event Burn(address indexed _burner, uint256 _burned);

        // Satisfies ERC 5679
        function burn(address _from, uint256 _amount, bytes calldata _data) external;

        // Burn given amount of tokens held by signer.
        function burn(uint256 _amount) external returns (bool);

        // Burn all tokens held by signer.
        // Returns the amount of tokens burned.
        function burn() external returns (uint256);

        // Total amount of tokens that have been burned.
        function totalBurned() external returns (uint256);

        // Total amount of tokens ever minted.
        // If totalSupply() is available (ERC20, ERC721 Enumerable), this equals totalSupply() + totalBurned().
        function totalMinted() external returns (uint256);
    }

#### Example implementation

<https://git.grassecon.net/cicnet/erc20-demurrage-token.git>

### Chrono

Define a creation time for a resource.

Complements `ERC5007`.

#### ERC165 Interface identifier

4db1ccd4

#### Solidity interface definition

    interface IChrono {
        // Returns the timestamp of when a resource corresponding to _idx was first created.
        // int64 chosen as return value for simpler interoperability with ERC5007.
        function createTime(uint256 _idx) external returns(int64);
    }

#### Example implementation

<https://git.defalsify.org/eth-erc721> (BadgeToken contract)

### Declarator

Permissionless store of signed claims made by an address about other
addresses, or addresses about themselves.

It is used to declare or respond to certifications of vouchers, NFT,
voucher members.

Addresses may be Externally Owned Accounts or smart contracts.

Claims may be made with or without topics. A missing topic is synonymous
with a topic value of `bytes32(0x00)`.

Any number of claims can be made about an address under any number of
topics. All claims must be stored, and returned in the order which they
were added.

#### ERC165 Interface identifier

21b7493b

#### Solidity interface definition

    interface IDeclarator {
        event DeclarationAdded(address indexed _declarator, address indexed _subject, bytes32 indexed _topic, bytes32 _proof);
        
        // Get all declarations for a subject (without topic) signed by a declarator
        function declaration(address _declarator, address _subject) external view returns ( bytes32[] memory );

        // Get all declarations for a subject for the given topic signed by a declarator
        function declaration(address _declarator, address _subject, bytes32 _topic) external view returns ( bytes32[] memory );

        // Get number of declarations the declarator has ever signed
        function declarationCount(address _declarator) external view returns ( uint256 );

        // Get the subject of a declarator's declarations at the specific index
        function declarationAddressAt(address _declarator, uint256 _idx) external view returns ( address );

        // Add a declaration for the subject
        function addDeclaration(address _subject, bytes32 _proof) external returns ( bool );

        // Add a declaration with topic for the subject
        function addDeclaration(address _subject, bytes32 _proof, bytes32 _topic) external returns ( bool );

        // Get the declarator that signed a declaration at the specificed index for a subject
        function declaratorAddressAt(address _subject, uint256 _idx) external view returns ( address );

        // Get the number of declarators that have signed for a subject
        function declaratorCount(address _subject) external view returns ( uint256 );
    }

#### Reference implementation

<git://holbrook.no/eth-address-index.git>

### Digest

Allows encoding of digests according to a specific encoding scheme.

Primary use-case is the abstraction of self-describing
[Multhash](https://multiformats.io/multihash/) encoding.

A default encoding *must* always be defined, and the encoding of a valid
digest *must* succeed with the default encoding.

#### ERC165 Interface identifier

982ab05d

#### Solidity interface definition

    interface IDigest {
        // Returns the default digest encoding used by the contract instance.
        function defaultDigestEncoding() external view returns (uint256 _encoding);

        // Check if the given encoding has been implemented in the contract instance.
        function haveDigestEncoding(uint256 _codec) external view returns(bool);

        // Verify and encode the given digest for a specific hashing algorithm.
        // Returns a zero-length byte array if digest is invalid.
        // Must succeed if called with the defaultDigestEncoding and a valid digest.
        function encodeDigest(bytes memory _data, uint256 _encoding) external view returns (bytes memory);

        // Encodes the digest using the default digest encoding.
        // Returns a zero-length byte array if digest is invalid.
        // Must succeed with a valid digest.
        function encodeDigest(bytes memory _data) external view returns (bytes memory);
    }

#### Example implementation

<git://holbrook.no/eth-event-msg.git>

### Expire

Defines an expiry time after which token balances and supply *cannot
change*.

A contract defining an expiry *must not* allow changing the expiration
time to a time in the past.

#### ERC165 Interface identifier

841a0e94

#### Solidity interface definition

    interface IExpire {
        // Contract has expired.
        event Expired(uint256 _timestamp);

        // Expiry time has changed.
        event ExpiryChange(uint256 indexed _oldTimestamp, uint256 _newTimestamp);

        // The current expiration timestamp.
        function expires() external returns (uint256);

        // Check expiry and apply expiration if expired.
        // Return values must be:
        // 0: not yet expired.
        // 1: already expired.
        // >1: expiry executed.
        function applyExpiry() external returns(uint8);
    }

#### Example implementation

<https://git.grassecon.net/cicnet/erc20-demurrage-token.git>

### Faucet

Used for dispensing tokens to any address.

It can be used for gas tokens and *ERC20* alike.

The interface is the same whether the faucet is dispensing from existing
balance or minting new tokens.

The value dispersed *must* be the same for all addresses.

In general, four criteria are expected to exist in any combination for
limiting access to the faucet:

Time  
A recipient may only use the faucet again after some time has passed.

Balance threshold  
A recipient may only use the faucet after its balance is below a certain
amount.

Membership  
A recipient may only use the faucet if it has been added to an access
control list.

Capacity  
The contract has sufficient token funds to dispense the current defined
amount to dispense.

The *check(address)* contract call *must* evaluate all four criteria,
and *must* return `false` if any of the criteria are not met.

#### ERC165 Interface identifier

1a3ac634

#### Solidity interface definition

    interface IFaucet {
        // Tokens were given to an address
        event Give(address indexed _recipient, address indexed _token, uint256 _value);

        // The amount that the faucet disperses has changed
        event FaucetAmountChange(uint256 _value);

        // Address of token the faucet represents
        // The faucet will return gas tokens with the zero-address is returned.
        function token() external returns (address);

        // Amount of tokens the faucet gives out
        function tokenAmount() external returns (uint256);

        // Give tokens to the given recipient. Returns amount of tokens given.
        function giveTo(address _recipient) external returns (uint256);

        // Give tokens to yourself. Returns amount of tokens given.
        function gimme() external returns (uint256);

        // Check if faucet may be used in the current contract state by _recipient
        function check(address _recipient) external view returns (bool);

        // Returns timestamp when faucet may be used again by _recipient
        // If 0 is returned, the address has not yet been used.
        // A return value of max(uint256) indicates that the faucet may not be used again.
        function nextTime(address _recipient) external returns (uint256);

        // Returns the token balance under which faucet may be used again by _recipient
        // A return value of max(uint256) indicates that the faucet may be used regardless
        //   of the token balance of _recipient
        function nextBalance(address _recipient) external returns (uint256);
    }

#### Reference implementations

- <git://holbrook.no/erc20-faucet.git>

- <git://holbrook.no/eth-faucet.git>

### Locator

This interface supports `ERC721 Metadata`, in particular the
`tokenURI(uint256)` call.

Off-chain resources in the CIC network *must* be defined in terms of
content addressed strings.

It *must* be possible to refer to all off-chain resources directly by
the content address.

Furthermore, it *should* be possible to refer to a resource by a
fully-qualified location on the web or an overlay network (e.g. tor).

#### ERC165 Interface identifier

ed75b333

#### Solidity interface definition

    interface ILocator {
        // URI that may or may not point to a specific resource location.
        function toURI(bytes memory _data) external view returns (string memory);

        // URL pointing to a specific resource location.
        function toURL(bytes memory _data) external view returns(string memory);
    }

#### Example implementation

<git://holbrook.no/eth-event-msg.git>

#### Expressing locators in terms of numeric token id

Given the numeric token id `1234567890987654321` (`0x112210f4b16c1cb1`
hex), and a base url `https://contentgateway.grassecon.net`, the result
of the methods may be as follows:

`toURI(toHex(1234567890987654321))`  
-\>
`https://contentgateway.grassecon.net/000000000000000000000000000000000000000000000000112210f4b16c1cb1`

`toURL(toHex(1234567890987654321))`  
-\>
`https://contentgateway.grassecon.net/000000000000000000000000000000000000000000000000112210f4b16c1cb1`

`tokenURI(1234567890987654321)`  
-\>
`https://contentgateway.grassecon.net/000000000000000000000000000000000000000000000000112210f4b16c1cb1`

#### Expressing locators in terms of a digest

Given the data `foo`, the digest algorithm `sha256` and a base url
`https://contentgateway.grassecon.net`, the result of the methods may be
as follows:

`toURI(sha256(foo))`  
-\>
`"sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"`

`toURL(sha256(foo))`  
-\>
`"https://contentgateway.grassecon.net/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"`

`tokenURI(toUint(sha256(foo)))`  
-\>
`"https://contentgateway.grassecon.net/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"`

#### Locator without URL

Given the data `foo`, the digest algorithm `sha256` and no base url, the
result of the methods may be as follows:

`toURI(sha256(foo))`  
-\>
`"sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"`

`toURL(sha256(foo))`  
-\> `""`

`tokenURI(toUint(sha256(foo)))`  
-\>
`"sha256:2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"`

### Minter

Attached to `ERC20` and `ERC721` tokens that may be minted.

Implements the `mint(...)` and `safeMint(...)` parts of `ERC5679` for
interoperability.

#### ERC165 Interface identifier

5878bcf4

#### Solidity interface definition

    interface IMinter {
        // Tokens are successfully minted; by who, to whom and how much
        event Mint(address indexed _minter, address indexed _beneficiary, uint256 value);

        // The given token has been successfully minted; by who, to whom and how much
        event Mint(address indexed _minter, address indexed _beneficiary, address indexed _token, uint256 value);

        // Mint the specified value of tokens to the specified recipient
        function mintTo(address _beneficiary, uint256 value) external returns (bool);

        // Satisfies ERC5679 for ERC20
        function mint(address _beneficiary, uint256 value, bytes calldata _data) external;

        // Satisfies ERC5679 for ERC721
        function safeMint(address _beneficiary, uint256 value, bytes calldata _data) external;
    }

#### Example implementation

<https://git.grassecon.net/cicnet/erc20-demurrage-token.git>

### Msg

Enables a reference "message" to describe the contract using an
off-chain resource.

The reference may or may not be mutable.

The interface complements `Locator` and `MultiHash` to generate locators
for how to resolve the reference.

#### ERC165 Interface identifier

a3002595

#### Solidity interface definition

    interface IMsg {
        // Emitted when a new message digest has been set
        // Should not be emitted if the digest set is identical to the previous
        event Msg(bytes _msgDigest);

        // Get the current message content hash
        function getMsg() external view returns(bytes memory);
    }

#### Example implementation

<git://holbrook.no/eth-event-msg.git>

### Registry

The Registry interface is a key-value store resolving well-known
contract identifier names to contract addresses.

It currently has two distinct uses in the CIC context:

1.  Entry-point to discover all relevant contracts of CIC networks.

2.  Unique (ERC20) token symbol resolver.

#### ERC165 Interface identifier

effbf671

#### Solidity interface definition

    interface IRegistryClient {
        // Address added to store with the given key
        event AddressKey(bytes32 indexed _key, address _address);

        // Return the address of the contract identified by the given byte string
        function addressOf(bytes32) external view returns (address);

        // Indexed accessor for the full list of registred identifiers
        function identifier(uint256) external view returns (bytes32);

        // Number of registered interfaces
        function identifierCount() external view returns (uint256);
    }

#### Contract registry implementation

<git://holbrook.no/eth-contract-registry.git>

#### Token index implementation

<git://holbrook.no/eth-contract-registry.git>

### Seal

Some smart contract parameters may need to be mutable over part of a
smart contract’s lifetime.

This interface provides a method to explicitly signal when certain
parameters have been rendered immutable.

The value of `sealState()` *must not* decrease, and must not exceed
`maxSealState`.

`maxSealState` is used to define that *all mutable parameters* have been
rendered immutable. The practical implications of this will vary between
contracts.

The implementer is encouraged to use simple, descriptive names in the
source code to describe the applicable seal states.

Use cases of sealing include:

- Whether more tokens can be minted

- Allow ownership of a contract to be transferred

- The expiry time of a token (see `Expire`)

#### ERC165 Interface identifier

0d7491f8

#### Solidity interface definition

    interface ISeal {
        // Seal state has changed.
        event SealStateChange(bool indexed _final, uint256 _sealState);

        // The current seal state.
        function sealState() external view returns(uint256);

        // The numeric seal state in everything sealable has been sealed.
        function maxSealState() external view returns(uint256);
    }

#### Example implementation

<https://git.grassecon.net/cicnet/erc20-demurrage-token.git>

### TokenLimit

Define limits of value amounts of tokens that individual addresses can
hold.

Limits are inclusive; a limit for 42 means transfer resulting in a token
balance *higher* than 42 should be rejected.

A return value of 0 indicates that the token is categorically not
accepted by the holder.

#### ERC165 Interface identifier

23778613

#### Solidity interface definition

    interface ITokenLimit {
        // Returns limit of total value a holder will accept of a specific token.
        // The value limit returned is inclusive; A limit of 42 means any operation resulting in a balance OVER 42 should be rejected.
        // A value of 0 means that no value of the token is accepted.
        function limitOf(address _token, address _holder) external view returns(uint256);
    }

#### Reference implementations

- <git://holbrook.no/erc20-limiter.git>

### TokenQuote

Quote an output token value for a given value of input tokens.

Both input and output value is denominated in the smallest available
unit of respective tokens.

#### ERC165 Interface identifier

dbb21d40

#### Solidity interface definition

    interface ITokenQuote {
        // Returns, within a current context, what value of outTokens the given value of inTokens translates to.
        // The values are given in smallest unit of each respective token.
        function valueFor(address _outToken, address _inToken, uint256 value) external view returns (uint256);
    }

#### Example implementation

- <git://holbrook.no/erc20-limiter.git>

### TokenRateChange

Enables detection of properties for CIC tokens that change value over
time, e.g. through demurrage.

It allows defining the granularity of the rate of change, aswell as a
frequency for which the rate of change is applied.

A method for canonical value change calculations is also provided.

#### ERC165 Interface identifier

b716af03

#### Solidity interface definition

    interface ITokenChangeRate {
        // Time unit resolution for rate of change.
        // A value of 0 indicates no rate of change,
        function changeTimeUnit() external view returns(uint256);

        // Rate of change per changeTimeUnit(), signed integer.
        // An effective value of 0 indicates no rate of change.
        function changeRate() external view returns (int256);

        // Number of decimals with which the changeRate is defined.
            // changeRate() should be divided by 10 ** changeRateDecimals() to obtain effective value
        function changeRateDecimals() external view returns(uint8);

        // Timestamp from when rate of change should be applied.
        // A value of 0 indicates no rate of change,
        function changeStartTime() external view returns(uint256);

        // Calculate value with change rate applied for given amount of time units.
        // Will calculate compounded change over the given amount of time units
        function applyChange(int256 _value, uint256 _changeTimeUnits) external view returns(int256);
    }

#### Reference implementations

- <git://holbrook.no/erc20-demurrage-token.git>

### TokenSwap

Token swap interface that can fit token escrow purposes aswell as token
swap contracts.

Optionally may define a `defaultToken`, that may among other things be
used for `withdraw()` calls without an `inToken`.

An explicit *ERC20 approval* of the balance to be transacted will
usually be required for the implementing contract.

If the value 0 is passed as argument to `deposit` or `withdraw`,
implementers can choose one of two possible outcomes:

- Noop. 0 *must* be returned.

- Use *balance* or the max available *allowance*. The actual (spent)
  value *must* be returned.

#### ERC165 Interface identifier

4146b765

#### Solidity interface definition

    interface ITokenSwap {
        // Emitted when a new deposit has been made.
        event Deposit(address indexed _token, uint256 _value);

        // Default token used to access the token swap.
        // Returns zero-address if no default token is defined.
        function defaultToken() external returns (address);

        // Add inToken liquidity to the tune of given value. 
        // Requires token approval for the corresponding value.
        // If value is 0, up to the full approval MAY be used for the transfer.
        function deposit(address _inToken, uint256 _value) external returns (uint256);

        // Withdraw pending outToken balance of given value in the pool for the sender.
        // May require token approval for defaultToken if used by contract as exchange for the withdrawal.
        // If value is 0, up to the full approval value MAY be used for the transfer.
        function withdraw(address _outToken, uint256 _value) external returns (uint256);

        // Exchange inToken equalling given value for outToken.
        // Requires token approval for the value of inToken.
        // If value is 0, up to the full approval value MAY be used for the transfer.
        function withdraw(address _outToken, address _inToken, uint256 _value) external returns (uint256);
    }

#### Reference implementations

- <git://holbrook.no/erc20-pool.git>

#### Handling deposits

The implementation is free to decide whether deposits can be recovered
directly, or if they will be locked in the smart contract - temporarily
or permanently.

For the duration deposits are locked, they are part of the smart
contract’s swap liquidity. During this time, locked deposits may only be
withdrawn in exchange for other tokens accepted by the contract.

Deposits that are not locked should be withdrawable using the
`withdraw(address,uint256)` (`0xf3fef3a3`) method signature.

Of course, as with most swap contracts, the tokens available for
withdrawal by the holder may not necessarily match the tokens that were
deposited by the holder.

### TokenVote

Execute elections with granular ERC20 token votes.

A proposal submitted for vote may or may not contain multiple options.
If multiple options are available, an ERC20 token holder may distribute
its vote among the options with the granularity of the token balance.

Voted tokens **SHOULD** be locked until the voting has finalized.

Finalization of voting should be callable by anyone.

#### ERC165 Interface identifier

f2e0bfeb

#### Solidity interface definition

    interface ITokenVote {

        // A new proposal has been created.
        event ProposalAdded(uint256 indexed _blockDeadline, uint256 indexed voteTargetPpm, uint256 indexed _proposalIdx);

        // A proposal vote has been completed.
        // The proposal is identified by the serial number in _proposalIdx. It is up to the implementer to define how the proposal should be retrieved by index.
        // The proposal result may be in one of three states:
        // * Ratified (_cancelled == false, _insufficient == false)
        // * Cancelled (_cancelled == true, _insufficient == false)
        // * Not reached quorum (_cancelled == false, _insufficient == true)
        event ProposalCompleted(uint256 indexed _proposalIdx, bool indexed _cancelled, bool indexed _insufficient, uint256 _totalVote);

        // Propose a new vote.
        // Voting is active until one of:
        // * total cancel vote reach quorum (_targetVotePpm, ppm = parts-per-million).
        // * _blockWait blocks from now.
        function propose(bytes32 _description, uint256 _blockWait, uint24 _targetVotePpm) external returns (uint256);

        // Add a voting option to a proposal
        function addOption(uint256 _proposalIdx, bytes32 _description) external;

        // Get number of options available for the proposal.
        // This decides the boundary of the index that can be used with voteOptions(...)
        // If the result is 0, vote(...) can be used aswell.
        function optionCount(uint256 _proposalIdx) external view returns(uint256);

        // Get proposal option. Assumes that proposal was created with proposeMulti(...)
        function getOption(uint256 _proposalIdx, uint256 _optionIdx) external view returns (bytes32);

        // Get vote count for the given option.
        // If proposal has no options, it should be called with _optionIdx = 0
        function voteCount(uint256 _proposalIdx, uint256 _optionIdx) external view returns(uint256);

        // Vote on a proposal without options.
        // Assumes that proposal was created with propose(...) and will fail otherwise.
        function vote(uint256 _value) external returns (bool);

        // Vote on a proposal option. Assumes that proposal was created with proposeMulti(...).
        // Must work with a non-option proposal if _optionIndex is 0.
        function voteOption(uint256 _optionIndex, uint256 _value) external returns (bool);

        // Vote to cancel a proposal.
        // If cancel has the majority:
        // * A vote without options will have rejected the proposal description.
        // * A vote with options will have rejected the proposal description as well as all option descriptions.
        function voteCancel(uint256 _value) external returns (bool);

        // Finalize the vote for a proposal.
        // May be called if deadline has been passed, or if:
            // * quorum has been reached with cancel votes.
        // * quorum has been reached and proposal has no/only one option.
        function finalize() external returns (bool);
    }

#### Reference implementations

- <git://holbrook.no/evm-tokenvote.git>

### Writer

A complement to ERC173, which allows definition of a class of
super-users for a contract.

A super-user address may perform *more* actions than a "normal" address,
aswell as *some* actions normally limited to the *contract owner*.

If an *contract owner* is defined, No super-user should be able to
perform actions that *contract owner* cannot perform.

Typically, only the *contract owner*, if it is defined, can add or
remove a super-user.

Some use-case examples of super-user actions include:

- Mint new tokens.

- Change the amount dispensed by the faucet.

- Edit access control lists.

#### ERC165 Interface identifier

abe1f1f5

#### Solidity interface definition

    interface IWriter {
        // A writer has been added by _executor
        event WriterAdded(address _writer);

        // A writer has been removed by _executor
        event WriterDeleted(address _writer);

        // Add a new writer to the contract.
        function addWriter(address _writer) external returns (bool);

        // Remove existing writer from the contract.
        function deleteWriter(address _writer) external returns (bool);

        // Check whether the given address is a writer.
        function isWriter(address _writer) external view returns (bool);
    }

#### Example implementation

<https://git.grassecon.net/cicnet/erc20-demurrage-token.git>