Protocol Token Printing Permissions

1. Summary
GebPrintingPermissions allows governance to whitelist multiple independent GEBs to print the same protocol token. It imposes delays for adding and removing permissions and it can also allow specific systems to print tokens indefinitely. 
2. Contract Variables & Functions
Variables
authorizedAccounts[usr: address] - addresses allowed to cover or uncover systems
allowedSystems[system: address] - data about a system that is allowed to print protocol tokens
usedAuctionHouses[house: address] - mapping that shows whether a DebtAuctionHouse is allowed to print tokens or not
unrevokableRightsCooldown - minimum time (in seconds) after which as system will have indefinite permission to print protocol tokens
denyRightsCooldown - cooldown that must pass between calling startUncoverSystem and endUncoverSystem
addRightsCooldown - cooldown for quickly removing cover from a freshly added system 
coveredSystems - the current number of systems covered by the protocol token
protocolTokenAuthority - the authority contract dictating who can print tokens
AUCTION_HOUSE_TYPE - debt auction house identifier
Data Structures
SystemRights - struct that stores data about each AccountingEngine (system). Contains:
covered - whether this system is covered or not
revokeRightsDeadline - deadline after which governance can no longer remove printing permissions from the system
uncoverCooldownEnd - cooldown after which governance can call endUncoverSystem and uncover this system
withdrawAddedRightsDeadline - cooldown during which governance can uncover a system without having to wait until uncoverCooldownEnd passed
previousDebtAuctionHouse - the previous DebtAuctionHouse connected to the AccountingEngine
currentDebtAuctionHouse - the current DebtAuctionHouse connected to the AccountingEngine
Modifiers
isAuthorized - checks whether an address is part of authorizedAddresses.
Functions
addAuthorization(usr: address) - add an address to authorizedAddresses.
removeAuthorization(usr: address) - remove an address from authorizedAddresses.
modifyParameters(parameter: bytes32, data: uint256) - modify a uint256 parameter
giveUpAuthorityRoot() - give up the perinting permission's position of root inside the ProtocolTokenAuthority
giveUpAuthorityOwnership - give up the perinting permission's position of owner inside the ProtocolTokenAuthority
coverSystem(accountingEngine: address) - cover a new system / allow the DebtAuctionHouse set inside accountingEngine to print protocol tokens
startUncoverSystem(accountingEngine: address) - start to uncover a system / withdraw the permission of the previous and current DebtAuctionHouses that were associated with accountingEngine
abandonUncoverSystem(accountingEngine: address) - abandon the uncovering of a specific system
endUncoverSystem(accountingEngine: address) - end the uncovering process for a system
updateCurrentDebtAuctionHouse(accountingEngine: address) - update the currentDebtAuctionHouse of a specific system and set the old current auction house as the 
previousDebtAuctionHouse
removePreviousDebtAuctionHouse(accountingEngine: address) - remove the permission to print protocol tokens from an accountingEngine's previousDebtAuctionHouse
proposeIndefinitePrintingPermissions(accountingEngine: address, freezeDelay: uint256) - propose a deadline after which the accountingEngine / system cannot be denied printing permissions
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 uint256 parameter is updated.
GiveUpAuthorityRoot - emitted when giveUpAuthorityRoot is called.
GiveUpAuthorityOwnership -  emitted when giveUpAuthorityOwnership is called.
RevokeDebtAuctionHouses - emitted when both the current and the previous DebtAuctionHouses are denied printing permissions. Contains:
accountingEngine - the accounting engine whose debt auction houses are denied permissions
currentHouse - the current debt auction house
previousHouse - the previous debt auction house
CoverSystem - emitted when a new system is covered / allowed to print tokens. Contains:
accountingEngine - the accounting engine from the system that is being covered
debtAuctionHouse - the debt auction house set inside the accountingEngine
coveredSystems - the new number of systems being covered
withdrawAddedRightsDeadline - the deadline after which governance can no longer quickly uncover a system without calling startUncoverSystem and endUncoverSystem
StartUncoverSystem - emitted when governance starts to uncover a system. Contains:
accountingEngine - the address of the accounting engine whose debt auction houses are uncovered
currentDebtAuctionHouse - the current debt auction house connected to the accounting engine
previousDebtAuctionHouse - the previous debt auction house that was connected to the accounting engine
coveredSystems - the latest amount of covered systems
revokeRightsDeadline - the deadline after which governance will not be able to remove this system's printing rights
uncoverCooldownEnd - the deadline after which governance can call endUncoverSystem and finish the uncover process
withdrawAddedRightsDeadline - cooldown during which governance could have uncovered the system without having to call endUncoverSystem
AbandonUncoverSystem - emitted when governance abandons the uncover process for a system. Contains:
accountingEngine - address of the accounting engine who's no longer being uncovered
EndUncoverSystem - emitted when the uncovering process is finished. Contains:
accountingEngine - the address of the accounting engine whose debt auction houses are denied printing permissions
currentHouse - the current debt auction house connected to the accounting engine
previousHouse - the previous debt auction house that was connected to the accounting engine
UpdateCurrentDebtAuctionHouse - emitted when the current debt auction house of an accounting engine is updated. Contains:
accountingEngine - the address of the accounting engine whose current debt auction house is updated
currentHouse - the address of the new current debt auction house
previousHouse - the address of the previous debt auction house
RemovePreviousDebtAuctionHouse - emitted when the previous debt auction house is removed from a covered system. Contains:
accountingEngine - the address of the accounting engine whose previous debt auction house is updated
currentHouse - the address of the new current debt auction house
previousHouse - the address of the previous debt auction house
ProposeIndefinitePrintingPermissions - emitted when governance proposes a deadline after which they can no longer remove a system's permission to print tokens. Contains:
accountingEngine - the address of the accounting engine that's part of the system which will no longer be denied printing permissions
freezeDelay - the delay from the current timestamp after which the system will have indefinite printing permissions
3. Walkthrough
Current and Previous Debt Auction Houses
Inside the printing permissions contract, a system is represented by an AccountingEngine. The AccountingEngine has one debt auction house address in it at any time. That's the currentDebtAuctionHouse although governance may have previously had another auction house that was buggy or had to be upgraded and so the printing permissions keep track of both the currentDebtAuctionHouse and previousDebtAuctionHouse until there's no outstanding auction left in the previousDebtAuctionHouse.
Function Descriptions
coverSystem : checks that the debt auction house associated with that AccountingEngine is not already covered. Updates the withdrawAddedRightsDeadline for that specific AccountingEngine. The deadline makes sure that governance can withdraw cover from the AccountingEngine without the need to wait between startUncoverSystem and endUncoverSystem
startUncoverSystem: it cannot uncover a system if either the current or the previous debt auction house associated with that system have outstanding debt auctions to settle. It cannot uncover a system if that system is already in the process of being uncovered. It can withdraw cover from a system either if there are at least 2 systems covered or, even if there are multiple systems covered, the withdrawAddedRightsDeadline for the system that governance wants to uncover was not passed. It must make sure that at least one system is protected at any time the exception being when all the systems added have not passed their respective withdrawAddedRightsDeadline s
abandonUncoverSystem : it can only abandon the uncover for a system that is in the process of being uncovered.
endUncoverSystem : it must check again that neither the current nor the previous debt auction house has any outstanding auctions left. Similar to startUncoverSystem , it can withdraw cover from a system either if there are at least 2 systems covered or, even if there are multiple systems covered, the withdrawAddedRightsDeadline for the system that governance wants to uncover was not passed (same exception to the rule applies as for startUncoverSystem).
updateCurrentDebtAuctionHouse: it cannot update the currentDebtAuctionHouse if the new auction house is the same as the current one. It also cannot update the current auction house if previousDebtAuctionHouse is non null. It must allow the new auction house to print tokens and set previousDebtAuctionHouse as the last currentDebtAuctionHouse
removePreviousDebtAuctionHouse : it cannot remove the permission of the previousDebtAuctionHouse to print protocol tokens unless there are no outstanding auctions in that auction house.
proposeIndefinitePrintingPermissions : it sets a time after which governance can never remove cover from a specific accounting engine (and thus it's associated debt auction house). updateCurrentDebtAuctionHouse and removePreviousDebtAuctionHouse should still work even if now > allowedSystems[accountingEngine].revokeRightsDeadline

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Contents

1. Summary

2. Contract Variables & Functions

3. Walkthrough

Current and Previous Debt Auction Houses

Function Descriptions