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An error type can conform to one or more of these new protocols to customize its behavior and representation. From an implementation standpoint, the protocol conformances are used to fill in the user-info dictionary in NSError to interoperate with the Cocoa error-handling system. There are a few outstanding problems with this implementation, although it is fully functional: * Population of the userInfo dictionary is currently eager; we should use user info providers on platforms where they are available. * At present, the Swift dynamic casting machinery is unable to unbox a _SwiftNativeNSError when trying to cast from it to (e.g.) an existential, which makes it impossible to retrieve the RecoverableError from the NSError. Instead, just capture the original error---hey, they're supposed to be value types anyway!---and use that to implement the entry points for the informal NSErrorRecoveryAttempting protocol. This is part (1) of the proposal solution.