it seems to me like there needs to be a new single letter tag put to use

there is fully 52 possible ones, each alphabetic letter in lower and upper case, and hardly any of them have been used

might take a while to scan all the PRs hanging on the repo to find what is being bagsed to try and avoid a conflicting semantic, should be fine to use the same letter as another with a different meaning in a different kind (or even syntax, such as multiple fields like title, author, isbn, etc