Proposed BIP for OP_CAT

In the email, Andrew Poelstra discusses his experience with rolling sha2 and the use of OP_MULTISHA256 in his templating investigations with OP_TXHASH. He mentions that he prefers OP_MULTISHA256 over OP_CAT, but acknowledges that OP_CAT allows for a Schnorr signature trick. He suggests considering the use of rolling sha2, but also mentions that he just learned about the 1000 element limit and couldn't find it in BIP-141 upon scanning.

To address the 1000 element limit, Andrew proposes a solution where instead of counting simple stack depth, each stack entry is counted as (1 + /520) entries. He explains that even though only 520 bytes can be pushed, these entries can still be made with OP_CAT. He notes that the stack and altstack variables in interpreter.cpp would need to be objects instead of vectors to count entries differently. However, he believes that implementing this logic would be simple enough and could be enabled unconditionally since it cannot be violated prior to using OP_CAT.

Overall, Andrew's email touches on his preference for OP_MULTISHA256 in his templating investigations, his acknowledgement of the 1000 element limit, and his proposal to address this limit by counting each stack entry differently.