foldMapComments :: Monoid b => (CommentId -> Maybe ChapterId -> Comment -> b)
                -> B.ByteString -> IO b
foldMapComments f =
    foldComments (\cId mChId c -> (f cId mChId c `mappend`)) mempty
foldMapComments :: Monoid b => (CommentId -> Maybe ChapterId -> Comment -> b)
                -> B.ByteString -> IO b
foldMapComments f =
    foldComments (\cId mChId c -> (f cId mChId c `mappend`)) mempty
foldMapComments f =
    foldComments (\cId mChId c -> (f cId mChId c `mappend`)) mempty