Proposed BIP for OP_CAT

The email discusses the confusion surrounding the enthusiasm for a "just CAT" softfork and questions its effectiveness. The sender mentions that there is no compelling example with working code that solely relies on CAT. They express confusion about the widespread support for a CAT-only fork and suggest that more opcodes, such as introspection opcodes or CHECKSIGFROMSTACK, would be needed for actually-interesting covenants. The sender believes that a CAT-based approach would be more circuitous compared to alternative approaches that have been discussed for years. They argue that CAT alone, without many auxiliary introspection opcodes, does not facilitate vaults that overcome usability hurdles described in a referenced paper. The sender provides an example of batched withdrawals and partial unvaultings that don't seem possible with just CAT. They also mention Burak's prototype, which was unable to handle revaulting, an important usability feature. The sender raises a question about the extent to which Taproot obviates this use case.