Programmatically access all the possible values of a defcustom

[Sebastien Vauban]

Hello,

Is there a possibility to programmatically access all the values that
a defcustom can take?

The example I have in mind is `org-babel-load-languages' which can
become any list with values such as:

--8<---------------cut here---------------start------------->8---
  (choice
   (const :tag "Awk" awk)
   (const :tag "C" C)
   (const :tag "R" R)
   (const :tag "Asymptote" asymptote)
   (const :tag "Calc" calc)
   (const :tag "Clojure" clojure)
   (const :tag "CSS" css)
   (const :tag "Ditaa" ditaa)
   ...
--8<---------------cut here---------------end--------------->8---

I'd like to list them, not the default value, but the full list of
languages. Any idea how to process?

Best regards,
  Seb

-- 
Sebastien Vauban