code: func (p Prefix) Above() []byte {
  	for i := len(p) - 1; i >= 0; i-- {
  		c := p[i]
  		if c < 0xff {
  			inc := make([]byte, i+1)
  			copy(inc, p)
  			inc[i]++
  			return inc
  		}
  	}
  	return nil
  }
docs: // Get the lexicographical sibling above this prefix (i.e. the fixed length integer plus one)