+* Consider storing git blob ID in Xapian doc data to avoid ref
+ and tree lookups based on Message-Id.
+
+* streaming Email::MIME replacement: currently we generate many
+ allocations/strings for headers we never look at and slurp
+ entire message bodies into memory.
+ (this is pie-in-the-sky territory...)
+
+* Allow in-place Xapian updates without clobbering the whole
+ index (versioning each doc data entry?) for big archives
+