* is_msg_is_mset() has been renamed to is_msg_in_sequence_set() because

* is_msg_is_mset() has been renamed to is_msg_in_sequence_set() because
  "sequence set" is now the official terminology as of RFC3501.
* imap_set_seen_flags() no longer calls is_msg_in_sequence_set() for each
  message and for each flag.  It's just too expensive.  We now parse each
  flag's sequence set manually, marking the relevant messages as we go.