code: func (b *bucket) Bucket(key []byte) database.Bucket {
  	// Ensure transaction state is valid.
  	if err := b.tx.checkClosed(); err != nil {
  		return nil
  	}
  
  	// Attempt to fetch the ID for the child bucket.  The bucket does not
  	// exist if the bucket index entry does not exist.
  	childID := b.tx.fetchKey(bucketIndexKey(b.id, key))
  	if childID == nil {
  		return nil
  	}
  
  	childBucket := &bucket{tx: b.tx}
  	copy(childBucket.id[:], childID)
  	return childBucket
  }
docs: // Bucket retrieves a nested bucket with the given key.  Returns nil if
  // the bucket does not exist.
  //
  // This function is part of the database.Bucket interface implementation.