Function update

update(card: Flashcard, answer: AnswerDifficulty, bucketMap: Map<Flashcard, number>, retiredBucket: number): void
Update step for the Modified-Leitner algorithm.
Parameters
- card: Flashcard
  
  a flashcard the user just saw
- answer: AnswerDifficulty
  
  the difficulty of the user's answer to the flashcard
- bucketMap: Map<Flashcard, number>
  
  represents learning buckets before the flashcard was seen. Maps each flashcard in the map to a nonnegative integer bucket number in the range [0...retiredBucket] inclusive. Mutated by this method to put card in the appropriate bucket, as determined by the Modified-Leitner algorithm.
- retiredBucket: number
  
  number of retired bucket. Must be an integer >= 0.
Returns void