He clarifies that an index of the Unspent Transaction Output (UTXO) set would also require a height. However, he suggests the possibility of using a composite reference instead. Fabian mentions that if a pre-agreed height is established, an index alone might suffice, which could be compatible with the use-cases Tom has in mind. He also brings up the concept of assumeutxo, indicating that combining it with a pre-agreed height could be interesting.Fabian then discusses the option of using a short hash as an alternative to a composite reference. However, he notes that Tom's current scheme may be more space efficient than this approach. The email concludes with Fabian signing off.In summary, Fabian acknowledges simplifying his previous message and clarifies that an index of the UTXO set would require a height. He suggests the possibility of using a composite reference or a short hash, but acknowledges that Tom's current scheme may be more space efficient.