Why is NULL not indexable?

I'm kinda of curious because if you look at
src/backend/access/nbtree/nbtree.c there are comments stating that you can
add NULLs to the index but mentions that you can't because:

"that's an artifact of the strategy map architecture chosen in 1986."

I can't work out what the 'strategy' bit refers to. All I can find in the
source code is references to tables of magic numbers. I guess what I really
want to know is, how hard would it be to fix?

(BTW, it seems there's an awful lot of duplicated code between all the index
types. Is this a historical accident or is there a reason?)

--
Martijn van Oosterhout <kleptog@svana.org>
http://svana.org/kleptog/
- Artificial Intelligence is the science of making computers that behave
- like the ones in the movies.