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object Animal {
def main(args: Array[String]) { new Animal[Awake].goToSleep }
}
class Animal[A <: AwakeOrAsleep] {
def goToSleep[B >: A <: Awake]: Animal[Asleep] = new Animal[Asleep]
def wakeUp[B >: A <: Asleep]: Animal[Awake] = new Animal[Awake]
}
sealed trait AwakeOrAsleep
trait Awake extends AwakeOrAsleep
trait Asleep extends AwakeOrAsleep
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