Stability Fee Treasury

1. Summary
The StabilityFeeTreasury is meant to allow other contracts or EOAs to pull funds (stability fees) in order to finance their operations. The treasury is set up as a secondaryReceiver in the TaxCollector. Anyone can periodically call the contract in order to recompute an optimal amount of fees that should be kept in the treasury and send any additional surplus to the extraSurplusReceiver.
2. Contract Variables & Functions
Variables
authorizedAccounts[usr: address] - addAuthorization/removeAuthorization - auth mechanisms
allowance[receiver: address] - total withdrawal allowance
pulledPerBlock[usr: address, blockNr: uint256] - total amount pulled by a user on a specific block
safeEngine - address of the SAFEEngine
systemCoin - system coin address (Coin.sol)
coinJoin - system coin adapter address
extraSurplusReceiver - address that receives extra unused funds from the treasury
treasuryCapacity - maximum amount of stability fees should be kept in the treasury
minimumFundsRequired - minimum amount of stability fees that must be kept in the treasury at all times
expensesMultiplier - multiplier for the expenses incurred since the last transferSurplusFunds call. Ensures there's a buffer of funds on top of the expected expenses until the next 
transferSurplusFunds call
surplusTransferDelay - delay between two transferSurplusFunds calls
expensesAccumulator - total expenses incurred since treasury deployment
accumulatorTag - latest snapshot of expensesAccumulator
latestSurplusTransferTime - last timestamp when transferSurplusFunds was called
contractEnabled - global settlement flag
Data Structures
Allowance - struct that specifies the total and the per-block withdrawal allowance for an address. Contains:
total - the total allowance (in RAD)
perBlock - the per-block allowance (in RAD)
Modifiers
isAuthorized - checks whether an address is part of authorizedAddresses (and thus can call authed functions).
accountNotTreasury - checks that the receiver account for treasury funds is not the treasury itself
Functions
modifyParameters() - authorized function for changing treasury parameters
disableContract() - disable the treasury and transfer all of its funds to theextraSurplusReceiver
joinAllCoins() - join any ERC20 system coins that the treasury has into internal coin form (SAFEEngine.coinBalance)
getAllowance(account: address) - get the total and the per-block allowance for a user
setTotalAllowance(account: address, rad: uint256) - authorized function that changes the total allowed amount an address can withdraw from the treasury
setPerBlockAllowance(account: address, rad: uint256) - authorized function that changes the amount that an address can withdraw from the treasury every block
giveFunds(account: address, rad: uint256) - governance controlled function that transfers stability fees from the treasury to a destination address
takeFunds(account: address, rad: uint256) - governance controlled function that transfers stability fees from a target address to the treasury
pullFunds(dstAccount: address, token: address, wad: uint256) - take funds from the treasury. Reverts if the caller is not allowed to withdraw or if their per-block allowance doesn't allow it
transferSurplusFunds() - recalculate the amount of stability fees that should remain in the treasury and transfer additional funds to the extraSurplusReceiver
Events
AddAuthorization - emitted when a new address becomes authorized. Contains:
account - the new authorized account
RemoveAuthorization - emitted when an address is de-authorized. Contains:
account - the address that was de-authorized
ModifyParameters - emitted when a parameter is updated.
DisableContract - emitted when the contract is disabled.
SetTotalAllowance - emitted when the total allowance for an address is set. Contains:
account - the account whose allowance was changed
rad - the new total allowance for the account
SetPerBlockAllowance - emitted when the per-block allowance for an address is set. Contains:
account - the account whose per-block allowance was changed
rad - the new total allowance for the account
GiveFunds - emitted when governance offers funds to an address. Contains:
account - the account that received funds
rad - the amount of funds that were given
expensesAccumulator - the sum of all the expenses incurred since the contract was deployed
TakeFunds - emitted when governance calls the treasury to take funds from an address. Contains:
account - the account the treasury takes system coins from
rad - the amount of (internal) system coins to take (expressed as RAD)
PullFunds - emitted when an approved address pulls funds from the treasury. Contains:
sender - the msg.sender that called pullFunds
dstAccount - the address that received funds
token - the address of the token being transferred (in this case it's always the address of the ERC20 system coin)
rad - the total amount of internal system coins transferred
expensesAccumulator - the sum of all the expenses incurred since the contract was deployed
TransferSurplusFunds - emitted when surplus funds above the optimum amount are sent to the accounting engine. Contains:
extraSurplusReceiver - the address that received the extra funds
fundsToTransfer - the amount of funds sent
3. Walkthrough
The treasury is funded by stability fees coming from the TaxCollector or by anyone who is willing to send system coins (either internally or as ERC20 tokens) to it. Governance can also use takeFunds to transfer internal system coins from a source address to the treasury.

Governance is in charge with setting up authorized addresses that can pullFunds out of the treasury (if their total or per-block allowance permits) as well as setting treasury parameters in order to determine the optimum amount of funds that should remain in the contract at any time. We define "optimum" as a multiplier (expensesMultiplier) of the latest expenses incurred by the treasury since the last transferSurplusFunds call. 
transferSurplusFunds is the way the treasury recalculates the amount of funds it should keep in reserves and transfers any surplus to the extraSurplusReceiver. Note that there is a surplusTransferDelay time delay between recalculating the optimum and transferring surplus out of the contract.
Sending Funds to extraSurplusReceiveror to the Treasury Itself
In case governance wants to send funds to extraSurplusReceiver using giveFunds, expensesAccumulator will not increase. In case an address wants to pullFunds and send them to the extraSurplusReceiver, pullFunds will revert.

pullFunds will silently fail if the dstAccount is the treasury contract itself, whereas giveFunds will revert.

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FSM Wrapper

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Contents

1. Summary

2. Contract Variables & Functions

3. Walkthrough