Eric Wong [off-list ref] wrote:

https://lore.kernel.org/lkml/_/text/help/ has other prefixes
like "dfn:" which might be helpful for search.

Btw, I'm hoping to expand that search functionality to be
available for both local/private mail and git code repos
while being network-transparent and capable of handling
multiple accounts.

mairix and notmuch both hit scaling problems for my personal
mail; so I'll be needing on something that can replace them.