How to add additional index_format sequences?

[Parke Bostrom <parke.bostrom@xxxxxxxxx>]

Hi all,

This is my first post to mutt-dev.  Please let me know if this is not
the right place to ask this question.

I want to extend Mutt to add an index_format sequence.
http://www.mutt.org/doc/manual/manual-6.html#index_format

I want to add a sequence that displays the address of the recipient.
I have numerous email addresses that all end up in the same Maildir,
and I want to be able to see (in the index) which of my email
addresses each message was sent to.  (Yes I know that some messages
will have multiple recipients, so deciding which recipient(s) to
display may be complicated.)

Can anyone please point me at the files/functions that implement the
index_format sequences?

Thanks!

Parke