: store(set, attr, flags)
: uid_store(set, attr, flags)
      Sends a STORE command to alter data associated with a message
      in the mailbox. the set parameter is a number or an array of
      numbers or a Range object. the number is a message sequence
      number (store) or a unique identifier (uid_store).
      The return value is an array of imap.rb"/Net::IMAP::FetchData.

      ex).
        p imap.store(6..8, "+FLAGS", [:Deleted])
        #=> [#<Net::IMAP::FetchData seqno=6, attr={"FLAGS"=>[:Seen, :Deleted]}>, #<Net::IMAP::FetchData seqno=7, attr={"FLAGS"=>[:Seen, :Deleted]}>, #<Net::IMAP::FetchData seqno=8, attr={"FLAGS"=>[:Seen, :Deleted]}>]

