in out send recv functions - immutable or stable?

Hi all,

When I look at the int4 functions (int4in, int4out, int4send, int4recv), 
they are all marked "immutable". Then again, when I look at the varchar 
functions, the in and out functions are immutable, but the send and 
receive functions are stable.

Is there a reason for this?
            Shachar

P.S.
This is PG 7.4 on Debian unstable, coming from the standard deb.

-- 
Shachar Shemesh
Lingnu Open Source Consulting ltd.
http://www.lingnu.com/