These rely on a feature called "Field Processors" that needs a pretty
new version of xapian, so it's optional. I haven't thought through all
the issues involved with having optional features in (lib)notmuch. I
guess it would make sense to have some API that tells you what
optional features are available. At least in this case there doesn't
seem to be any relationship with database features. We also need to
figure out how to document optional features; presumably it would be
helpful to tell people how to interrogate notmuch (CLI) to see if a
given feature is supported.

d