code: func (c *Change) Action() (merkletrie.Action, error) {
  	if c.From == empty && c.To == empty {
  		return merkletrie.Action(0),
  			fmt.Errorf("malformed change: empty from and to")
  	}
  	if c.From == empty {
  		return merkletrie.Insert, nil
  	}
  	if c.To == empty {
  		return merkletrie.Delete, nil
  	}
  
  	return merkletrie.Modify, nil
  }
docs: // Action returns the kind of action represented by the change, an
  // insertion, a deletion or a modification.