I should probably make clear at the outset that I really don't care about getting a down-vote, even one without a comment. I'm an adult, I can bear the shame and ignominy.
But I don't think that this is the type of answer that should garner down-votes, at least beyond 0. It was an answer that solved the question in good faith, but because I didn't know about the
\middle command, it was not the right answer. The point is that there are so many obscure commands and packages that no one person is going to know all about them. I'd rather people were encouraged to be helpful - as I was trying to be - than to hang back and wait to see if someone knows of a neat command already out there that does the job.
This isn't something to legislate on, but rather something that by our behaviour we can encourage.
(NB Regarding the time-line on that answer; when I started writing it, there were no other answers)