IIUC this string/replace takes a value like p123__456# and returns p123__#, and this works because the pX number is related to the place within the form. Did I get that right?

I am curious if the pX is local to the binding form, or to the file, or global. I could see any of them being reasonable. I think best for us would be local to the binding form, because it would mean edits to other places in the same file/project wouldn't bust the fast refresh cache. I think no matter what, this is better than what we had before!

The first number is the index of the parameter, yep. So % corresponds to p1, so does %1. %2 corresponds to p2 etc.

And obviously %& corresponds to rest.

So local to the function bindings.