def thread(algorithm, search_keys, charset)
      return thread_internal("THREAD", algorithm, search_keys, charset)
    end
As for #search(), but returns message sequence numbers in threaded
format, as a Net::IMAP::ThreadMember tree.  The supported algorithms
are:

ORDEREDSUBJECT:: split into single-level threads according to subject,
ordered by date.
REFERENCES:: split into threads by parent/child relationships determined
by which message is a reply to which.

Unlike #search(), +charset+ is a required argument.  US-ASCII
and UTF-8 are sample values.

See [SORT-THREAD-EXT] for more details.