Bitcoin's SIGHASH_ANYPREVOUT enables flexible rebindable transactions

SIGHASH_ANYPREVOUT (APO) is a proposed Bitcoin upgrade that allows a signature to authorise any compatible UTXO rather than one fixed outpoint. It enables rebindable pre-signed transactions useful for the Lightning Network, vaults and layer-2 protocols without introducing new key management overhead. The proposal is part of an ongoing series examining Bitcoin covenants.