can't train bogofilter

Hello, I have a mailbox folder with spam mails that bogofilter has
incorrectly filed as "Unsure" or "Ham", and I'd like to train it. So in
the directory I gave this command:

for i in $(ls); do bogofilter -Ns < $i; done

which takes a while, but after that all messages still have an
unmodified X-Bogosity header (i.e. it still reads "Unsure" or "Ham") -
is this the supposed behaviour? If I also give the -p option, the
message is written to stdout with a correct X-Bogosity header (i.e.
spam), but my bash-fu is weak and I can't figure out how to correctly
redirect this output to overwrite the original file - and most
importantly I don't know if that's really what I want to/should do.

Also, after that command the file ~/.bogofilter/wordlist.db hasn't
increased in size.

What am I missing?

cheers,
renato
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