Badly placed ()'s

Hi, i have this function:

DECLARE

cadena VARCHAR;
Libro ALIAS FOR $1;
numIni ALIAS FOR $2;
numFin ALIAS FOR $3;
res orden%ROWTYPE;
BEGIN

IF numIni <> 0 THEN

cadena := ''select * from orden where idOrden not in (select
idOrden from orden where idlibro = '';
cadena := cadena || Libro;
cadena := cadena || '' order by idorden limit '';
cadena := cadena || numIni;
cadena := cadena || '') and idlibro = '';
cadena := cadena || Libro;
cadena := cadena || '' order by idorden limit '';
cadena := cadena || numFin-numIni;
cadena := cadena || '';'';

ELSE

cadena := ''select * from orden where idlibro = '' || Libro ||
'' order by idorden limit '' || numFin || '';'';

END IF;
RAISE NOTICE ''%'',cadena;
FOR res IN EXECUTE cadena LOOP
RETURN NEXT res;
END LOOP;
return;
END;

And I get the error of badly placed ()'s... What is wrong?

Thanks

JuanF
'a very tired newbie'