code: func (tx Tx) Value(k [][]byte, f func(v bytesp.Slice) error) error {
  	return tx.Bucket(k[:len(k)-1], func(b Bucket) error {
  		v := b.Bucket.Get(k[len(k)-1])
  		if v == nil {
  			return nil
  		}
  		return errorsp.WithStacks(f(bytesp.Slice(v)))
  	})
  }
docs: // Value tries to get a value from the transaction. If the key does not
  // exist, the f is not called and nil is return.