- # if .tmp exists it is irrelevant
- # if .cdb exists it is a cdb database
- # containing exactly the same as .main
+ # .cdb is a cdb database containing data
+ # equivalent to and at least as recent as .main
+ # (maybe not identical, because .cdb may
+ # have been updated with data from .log but
+ # .main not yet); if .log does not exist then
+ # they are identical)
+ # .cdb may not exist; in which case it is to
+ # be treated as if it existed and was empty
+ # but this is maximally early (so main must
+ # exist and be empty since .main is never
+ # newer than .cdb)