# SystemDebtEngine

The `SystemDebtEngine` contract is responsible for managing the system's surplus and bad debt. It acts as the central financial manager within a FXD system, specifically designed to handle scenarios where debts become unmanageable or when excess funds are accumulated beyond what is required for the system's stability.

#### Storage Variables

* `bookKeeper`: Reference to the `IBookKeeper` contract, which handles the ledger of all financial operations within the system.
* `surplusBuffer`: The minimum amount of surplus (in RAD, 10^45) that should always be maintained in the system to cover unexpected liabilities.
* `live`: A flag indicating whether the system is active (1) or has been shut down (0).

#### Functions

* `withdrawCollateralSurplus(bytes32 _collateralPoolId, address _to, uint256 _amount)`: Allows the owner to withdraw surplus collateral from a specific collateral pool.
* `withdrawStablecoinSurplus(address _to, uint256 _value)`: Allows the owner to withdraw surplus stablecoins from the system, provided there is no outstanding bad debt and the withdrawal does not drop the balance below the surplus buffer.
* `setSurplusBuffer(uint256 _data)`: Sets the minimum surplus buffer required in the system.
* `settleSystemBadDebt(uint256 _value)`: Allows anyone to settle bad debt using the surplus funds available in the system.
* `cage()`: Can be called by the owner or a designated emergency role to shut down the system in case of a significant issue. It settles as much bad debt as possible with available funds during the shutdown.
* `pause()`: Pauses all critical functionalities of the contract, preventing state-changing operations.
* `unpause()`: Resumes all operations after a pause.

#### Events

* `LogSetSurplusBuffer(address indexed _caller, uint256 _data)`: Emitted when the surplus buffer is updated.
* `LogCage()`: Emitted when the system is shut down.