is prefix pg_ reservated ?

Hello all

I found strange behave of postgresql. I would to use name
pg_stat_get_some for custom function. Probably it is not smart from
me, because I found so functions that has prefix pg are not searched
via search_path.

postgres=# select public.pg_stat_get_creation_time('xxx'::regclass);  pg_stat_get_creation_time
-------------------------------2012-08-07 12:43:25.592137+02
(1 row)

postgres=# select pg_stat_get_creation_time('xxx'::regclass);
ERROR:  internal function "pg_stat_get_creation_time" is not in
internal lookup table

It is expected and desired behave?

Regards

Pavel Stehule