On May 17, 2016 15:23, "Peter Todd via bitcoin-dev" <
bitcoin-dev@lists.linuxfoundation.org> wrote:
> # TXO Commitments
>

> Specifically TXO commitments proposes a Merkle Mountain Range¹ (MMR), a
> type of deterministic, indexable, insertion ordered merkle tree, which
allows
> new items to be cheaply appended to the tree with minimal storage
requirements,
> just log2(n) "mountain tips". Once an output is added to the TXO MMR it is
> never removed; if an output is spent its status is updated in place. Both
the
> state of a specific item in the MMR, as well the validity of changes to
items
> in the MMR, can be proven with log2(n) sized proofs consisting of a
merkle path
> to the tip of the tree.

How expensive it is to update a leaf from this tree from unspent to spent?

Wouldn't it be better to have both an append-only TXO and an append-only
STXO (with all spent outputs, not only the latest ones like in your "STXO")?