> Uwe writes:

> Ah, yes, but setq doesn't define a quantity - it is much less abstract:
> it assigns a value to a variable.


> Thanks for explaining, I was genuinely puzzled; you are using a
> different definition of recursion than most people do, when it comes to
> programming - where it is usually, implicitly about functions.

I realised that there was a misunderstanding. I thought you and others
suggest just to use 

(defun my-new-insert-subject-counter-old ()
  "Insert a string of the form [`counter/Total'] "
  (interactive)
  (save-excursion
    (setq my-subject-counter (+ my-subject-counter 1))
    (let ((total (read-string "Total number: " )))
    (message-carefully-insert-headers (list (cons 'Subject (format "[%s/%s]" my-subject-counter total)))))))

Without 
(defvar my-subject-counter 0) (or something like this.)

As a matter of fact I deleted the defvar definition from my code and it
still worked[1] but which puzzled me.

So I thought 
    (setq my-subject-counter (+ my-subject-counter 1))

defines and sets the variable in a recursive way, but it does not. You
need a defvar (or a surrounding let). 

I only discovered my error misunderstanding when I restarted emacs and
the code did not work anymore

Sorry for the misunderstanding.


Footnotes:
[1]  because it was already defined