exceptions is implemented as a list, although it would be more
meaningful for it to be a set, but then there is a problem with
web.py:sync_db
-> maint.py:webbd_add_lexicons
-> sql_store.py:update_lexicon
-> lexical units are stored in the SQL database as their json dumps,
but the json encoder does not know how to serialize a set

json_utils.py contains a register decorator which probably could be used
for dealing with this, but I don't know how to use it and a list works
equally well anyway