Before we would separate categories with a "-" (single dash), but that
could lead to confusion because the field separator is a "--" (double
dash).  Now the separator is the plus sign.

What follows are instructions to update existing entries.  Apologies for
the inconvenience, as I am not aware of a better way to handle this
case.

* * *

Refactor all your USLS files (WARNING---only tested with Emacs 29):

+ BACK UP ALL YOUR FILES (unless you are already tracking them with git,
  in which case double-check the following changes before committing).

+ Go to the USLS directory (if there are subdirectories, repeat this
  step for each of them).

+ Make Dired editable with `C-x C-q`

+ Start recording a keyboard macro with `F3`, `C-x (` or `M-x kmacro-start-macro`.

+ Type `M-:` and evaluate:

  ```elisp
  (progn
    (goto-char (point-min))
    (if-let* ((end (search-forward-regexp "--\\([0-9A-Za-z_-]*-[0-9A-Za-z_-]*\\)--" nil t))
              (beg (search-backward-regexp "--\\([0-9A-Za-z_-]*-[0-9A-Za-z_-]*\\)--" nil t)))
        (let* ((string (buffer-substring-no-properties beg end))
               (rep (replace-regexp-in-string "-+?" "+" (string-trim string "--" "--"))))
          (delete-region beg end)
          (insert (concat "--" rep "--")))
      (keyboard-quit)))
  ```

+ After evaluating the above, go back to the top of the buffer and close
  the keyboard macro with `F4`, `C-x )` or `M-x kmacro-end-macro`.

+ Repeat the keyboard macro with `C-u 0 F4` (`C-u 0` is the same as
  `C-0`).  If `F4` is not available, use `C-u 0 C-x e`.

+ Confirm the changes with `C-c C-c`.

* * *

Update all references to the original file paths (requires the `wgrep`
package):

+ Type `M-:` `(lgrep "--\\([0-9A-Za-z_-]*-[0-9A-Za-z_-]*\\)--.*\\(txt\\|org\\|md\\)" "*" default-directory)`

+ Instead of `lgrep` you can use `rgrep` if you have subdirectories.

+ In the resulting grep buffer, type `C-x C-q` to make it editable.

+ Now re-use the keyboard macro that was defined earlier: `C-u 0 F4`.

+ Confirm the changes with `C-c C-c`.

Done!