+
+(define (roll-back-machine machine)
+ "Monadic procedure rolling back to the previous system generation on
+MACHINE. Return the number of the generation that was current before switching
+and the new generation number."
+ (let ((environment (machine-environment machine)))
+ ((environment-type-roll-back-machine environment) machine)))
+
+\f
+;;;
+;;; Error types.
+;;;
+
+(define-condition-type &deploy-error &error
+ deploy-error?
+ (should-roll-back deploy-error-should-roll-back)
+ (captured-args deploy-error-captured-args))