code: func (b Bucket) OpenBucket(folders [][]byte, f func(Bucket) error) error {
  	bb := b.Bucket
  	for _, fld := range folders {
  		bb = bb.Bucket(fld)
  		if bb == nil {
  			return nil
  		}
  	}
  	return errorsp.WithStacks(f(Bucket{bb}))
  }
docs: // Bucket retrieves a nested bucket by name. Returns nil if the bucket
  // does not exist. The bucket instance is only valid for the lifetime of
  // the transaction. folders can be empty, in which case the b itself is
  // sent to f.