Function maildir_flags 

pub(crate) fn maildir_flags<'a>(flags: impl Iterator<Item = Flag<'a>>) -> String

IMAP’s system flags as Maildir’s letters (https://cr.yp.to/proto/maildir.html), and the keywords kept, $Junk and $NotJunk, as Dovecot’s lowercase letters (maildir::KEYWORDS): what you or a mail client said of a message, which the spam filter learns from and the Porch heeds. reconcile keeps them in step.