Feature request: min/max for macaddr type

My use case: I was writing some tests where I just needed some inets and
macaddrs in a database table with certain properties. I tried to use the
following queries:

INSERT INTO inet_table (inet, ..) VALUES ((SELECT max(inet) + 1 FROM
inet_table), ...);
INSERT INTO macaddr_table (macaddr, ...) VALUES ((SELECT max(macaddr) +
1 FROM macaddr_table), ...);

This works fine for the inet type, but the macaddr failed this. I tried
adding some functions for macaddr_smaller/macaddr_larger to implement
the min/max. The code for it was very trivial, but then I got completely
lost in pg_proc.h/pg_aggregate.h. I fail to find the documentation for
this files as well, is there any reference I could use to finish my work?

--
Herwin Weststrate