Concrete MATT opcodes

He states that his current goal is to facilitate maximum experimentation and believes that it is safe to open Pandora's box in a sandbox environment to determine if it is empty. He acknowledges that concerns will need to be addressed when a soft-fork proposal is made and mentions the possibility of adding restrictions if necessary.Salvatore discusses the potential use cases of cross-input introspection, providing a link to some notes he drafted on how it could be used to implement eltoo-style replacement for lightning or arbitrary state channels when combined with ANYONECANPAY. He also mentions that cross-input introspection may not be needed if CCV+CHECKSIGFROMSTACK is available, as it would make the implementation easier.Furthermore, Salvatore addresses concerns raised about the recursivity of covenant opcodes and suggests that it could be artificially limited in CCV if desired, although it would restrict some use cases. He expresses his intention to write about why the fear of covenants might generally be unjustified in the future.Salvatore discusses the possibility of introducing more than a binary flag to encode subsets and suggests generalizing the index parameter so that it can refer to a group of inputs/outputs instead. He invites Antoine to expand further on this topic. He also asks for clarification on how these "tags" would interact with CHECKCONTRACTVERIFY, as he does not fully understand the use case.Salvatore finds the generic framework interesting for joinpool/coinpool and suggests that more generic introspection could be added with separate opcodes, although it may not fit well within the semantics of CCV. He mentions that he personally prefers OP_CHECKSIGFROMSTACK when thinking about constructions with CCV but acknowledges that either opcode would likely work.Finally, Salvatore briefly mentions the "DeferredCheck" framework commit and emphasizes the importance of taking care when designing new deferred checks to avoid any sort of quadratic validation cost. He notes that the DeferredChecks specifically added for CCV have negligible cost.Overall, Salvatore's email covers various topics related to his thoughts and ideas on facilitating experimentation, cross-input introspection, covenant opcodes, encoding subsets, generic introspection, and the importance of careful design in implementing new deferred checks.