Bugs:
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Soon:
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- memoize result of running ifile -q
- fix Xref header issue
- extend to nnimap
- replace training shell scripts with generalized elisp

Eventually:
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- add statistics counters
- replace use of message with nnheader-message?
- enable multi-way classification

Approximately never:
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- generalize for different spamfilters
- maybe use -k option?  NO, grows DB to huge size
