Should `a:` forward `z`? Better yet, we could accept a `Bz` where a `W` is currently required

[darosior]

Could be something we want to do eventually.

12:09 < darosior> Why doesn't the 'a:' wrapper "forward" the 'z' property? Sure TOALTSTACK <X> FROMALTSTACK does technically consume the top stack element, but it puts it back and as long as X is 'z' it should appear to the following OPs as if it didn't consume anything.
12:11 < darosior> I was thinking about this in the context of https://github.com/sipa/miniscript/pull/105#issuecomment-1074459067, but unfortunately it would only enable '0' to be 'Wzdu' which is not very interesting. Still, i wonder if the 'a:' wrapper should for some reason never be 'z'?
12:11 <@sipa> There are other properties missing around W and z. Every Bz could be automatically treated as a W directly too.
12:11 <@sipa> But I think those aren't all that useful in practice.
12:16 < darosior> I'll open an issue regarding 'z' and 'W' <-> 'Bz', if you don't mind. Probably not worth addressing now but there could be use case we missed
12:17 <@sipa> Counting Bz as W or Wz would be even better than passing through z through a.
12:17 < darosior> Oh, right
12:17 <@sipa> It might need a "wrapper" that involves no opcode, like and_v, to convert Bz to Wz
12:18 <@sipa> Alternatively, we could just permit and_b or_b and thresh to accept Bz args
12:18 <@sipa> In addition to W args.

Link to the discussion logs

[darosior]

Also, re-constructing Pieter's list from #105 showed the need for Wo sometimes. I don't see anything wrong with forwarding it as well. (EDIT) Actually, does it change the current assumption if the consumed stack element isn't the top stack element?