Marten Veldthuis pointed out on the mailing list that intentional
spoofing is something that the user should be told about.
(tag-name, search-specification). The database is responsible for
ensuring that the virtual tag is always consistent.
+Indicate to the user if two files with the same message ID have
+content that is actually different in some interesting way. Perhaps
+notmuch initially sees all changes as interesting, and quickly learns
+from the user which changes are not interesting (such as the very
+common mailing-list footer).
+
General
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Audit everything for dealing with out-of-memory (and drop xutil.c).