the "~" operator (was Re: Add id to next element)

Regarding

> I normally use (expr1, expr2)[1] as an if-then-else for expressions
evaluating to 0 or 1 node, but that arrow syntax is pretty neat. I should
start using it.

While researching the arrow operator, I came across Michael Kay's blog post
at

https://dev.saxonica.com/blog/mike/2020/11/19-arrow-expressions.html

and I want that ~ operator for within-expression and across-element streaming
soooo bad... I have so many xsl:variable chains I could clean up with that!

 - Chris