[O] Why is there no org-agenda-todo-next-keyword function?

[Somelauw .]

In an org-agenda buffer the following functions are defined and bound:
- org-agenda-todo-nextset (C-S-left)
- org-agenda-todo-previousset (C-S-right)
which change between sets of todo keywords.

Yet the following corresponding functions are missing:
- org-agenda-todo-next-keyword
- org-agenda-todo-previous-keyword
which should (if implemented) change between individual todo keywords.

That means that If I have the following setup:

```emacs-lisp
  (setq org-todo-keywords
        '((sequence "TODO(t)" "|" "DONE(d)")
          (sequence "NEXT(n)" "|" "WAITING(w)")))
```

There is no way to change a task to DONE using C-S-left and C-S-right
bindings alone.

What's the reason that these functions are missing?