additionally saving history for each directory?

[Greg Klanderman <gak@xxxxxxxxxxxxxx>]

I feel like somewhat recently someone posted a few line example of
saving per-directory history, but I've been unable to find it.
Somehow that popped into my head earlier, and seemed like a useful
feature to additionally be saving history across shells to
directory-specific history files, and then have alternate key bindings
of eg. history-incremental-search-backward and
history-beginning-search-backward that search the history for the
current directory instead of the shell instance's history.  Anyone
have a simple recipe for something like that to get me started?

thank you,
Greg