Extended BNF?

[Paul Hilfinger]

Has anyone considered adding extended BNF to Bison?  I refer to things
like

 A.     p : a? ;
 B.     p : a ( b | c ) d ;
 C.     p : a ( ',' a )*
 D.     p : a+

and so forth.  There are numerous questions to answer, mostly involving 
semantic actions and including:

1. How do we specify the semantic types of such constructs?
2. How do we refer to, e.g., the values of b and c from example B in actions?
   Presumably this would only be legal inside the parentheses.
3. Assuming we translate * and + into equivalent non-extended
   productions, how do we specify left vs. right-recursive lists?
4. How do we specify list constructors for * and +?
5. How do we supply a value for the empty case in A?
6. How do we supply a value for the ( ... ) construct in B?

Paul Hilfinger