Weird EXECUTE ... USING behaviour

In a PL/PgSQL function I have the following:

----
            execute $l2$
              alter table $l2$||ct||$l2$ add check(
data>=$1::timestamp and data<$2::timestamp and maga=$3 )
            $l2$ using rec.d0,rec.d1,rec.maga;
----
which yields to this error messsge:
----
ERROR:  there is no parameter $1
CONTEXT: SQL statement "
              alter table public.test_part_2 add check(
data>=$1::timestamp and data<$2::timestamp and maga=$3 )
            "
PL/pgSQL function "f_partition_test" line 25 at istruzione EXECUTE
----
whiile this fragment:
----
            execute $l2$
              insert into $l2$||ct||$l2$
                select * from only public.test
                where data>=$1::timestamp and data<$2::timestamp and maga=$3
            $l2$ using rec.d0,rec.d1,rec.maga;
----
is executed without a glitch in the very same function body.
Where's my error?


--
Vincenzo Romano
NotOrAnd Information Technologies
NON QVIETIS MARIBVS NAVTA PERITVS