As suggested by Keith in FLAG_PURE_NOT allows for expressions like:
notmuch search NOT tag:inbox
Note that this way a search like:
notmuch search foobar NOT tag:inbox
should not be written instead:
notmuch search foobar AND NOT tag:inbox
In my opinion, the latter feels more natural and is somewhat more explicit.
It gives a better clue of what the search is about instead of assuming that
an implicit AND operator is there.