[O] Change section (headings) numbering style

[Axel E. Retif]

Greetings!

I need to change the numbering style of the first level sections (*) in 
the exported html file.

Right now they are exported as

1 First section              ;; for * First section

1.1 First subsection         ;; for ** First subsection

etc.

For the first part of the document that's all right, but for the next 
part I need

A First new section          ;; for * First new section

A.1 First new subsection     ;; for ** First new subsection

and so on.

I've tried with different combinations of #+ATTR_HTML and #+OPTIONS with 
section-number-2 with no success.

TIA


Best,

Axel