How to cast to regprocedure with OUT parameters

Hello,

pg_get_functiondef() can be used quite easily by using a cast to regprocedure, like this:

select pg_get_functiondef('public.foo(text, text, text)'::regprocedure);

However if the function is defined with an out parameter like this:

create or replace function foo(p1 text, p2 out text, p3 text)
....

the above cast does not work. Postgres returns an error: "function public.foo(text,text,text) does not exist.

'public.foo(text, OUT text, text)'::regprocedure does not work either.

So, what should be the right format to write the string literal that can be cast to the correct regprocedure?

Thanks
Thomas