[Exim] How do the $header_ ($h_, etc..) variables work in th…

I'm trying to make a little patch that would allow accessing environment
variables via $env_<environment variable name>, so I was checking in
expand.c to see how the $header_<header name> stuff worked. I found the
code that does the actual extraction, but I can't figure out how/where the
$header_* stuff hooked in, since it's not like a normal variable (which I
could see how to create quite easily).

Can anyone give me some API references or just point me to a file/line so I
can see what I'd need to do to duplicate this?

Thanks!

BTW, If this is harder than I think - can someone add this ability to the
wishlist please? :) It'd be *very* useful in the non smtp acl to add custom
headers for emails sent by CGI type scripts which webusers usually use to
spam through (and since the script usually runs as a different user, it's
nearly impossible to track the spammer down on your own system).

Eli.

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