Adding new programlisting attributes

Hello,

In the docbook project some PI's are defined to define how line numbering  
should be done:
<?dbhtml linenumbering.everyNth="2" linenumbering.separator=" >"?>
<?dbfo linenumbering.everyNth="2" linenumbering.separator=" >"?>

Why not adding new official attributes to programlisting (and screen) to  
define these handy options? The XML code would then be less specific to a  
processing tool, and these attributes could be ignored by tools that  
cannot handle them.

What do you think?

Regards,
BG