
la a ::= Na | Ca a (lb a) ;

lb b ::= Nb | Cb b (la b) ; ;;





fa x = case x of
         Na -> 0;
         Ca y ys -> (fb ys) + y
       end;

fb x = case x of
         Nb -> 0;
         Cb y ys -> (fa ys) - y
       end;

