Claudio,

C> set_transaction : SET TRANSACTION tran_opt_list_m
C> {$$ = make_node (nod_trans, 1, make_list ($3)); }
C> ;

C> Why can't we extend this command instead of having to learn a different
C> syntax?

Personally, I did have a look at that and found it not suitable,
mainly for lack of an END statement. One way to use it would be to
have it act on whatever [compound] statement comes next, but I don't
think it's very intuitive.
--
Nando Dessena