This BIP is unnecessary, in my opinion.

I'm going to take issue with items (2) and (3) that are the motivation for this BIP:

" 2. Full nodes and SPV nodes following original consensus rules may not be aware of the deployment of a hardfork. They may stick to an economic-minority fork and unknowingly accept devalued legacy tokens."

If a hardfork is deployed by increasing the version number in blocks (as is done for soft forks), then there is no risk-- Full and SPV nodes should notice that they are seeing up-version blocks and warn the user that they are using obsolete software.

It doesn't matter if the software is obsolete because of hard or soft fork, the difference in risks between those two cases will not be understood by the typical full node or SPV node user.

" 3. In the case which the original consensus rules are also valid under the new consensus rules, users following the new chain may unexpectedly reorg back to the original chain if it grows faster than the new one. People may find their confirmed transactions becoming unconfirmed and lose money."

If a hard or soft fork uses a 'grace period' (as described in BIP 9 or BIP 101) then there is essentially no risk that a reorg will happen past the triggering block. A block-chain re-org of two thousand or more blocks on the main Bitcoin chain is unthinkable-- the economic chaos would be massive, and the reaction to such a drastic (and extremely unlikely) event would certainly be a hastily imposed checkpoint to get everybody back onto the chain that everybody was using for economic transactions.

Since I don't agree with the motivations for this BIP, I don't think the proposed mechanism (a negative-version-number-block) is necessary. And since it would simply add more consensus-level code, I believe the keep-it-simple principle applies.

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Gavin Andresen