def store(set, attr, flags)
      return store_internal("STORE", set, attr, flags)
    end
Sends a STORE command to alter data associated with messages
in the mailbox, in particular their flags. The +set+ parameter
is a number or an array of numbers or a Range object. Each number
is a message sequence number.  +attr+ is the name of a data item
to store: 'FLAGS' means to replace the message's flag list
with the provided one; '+FLAGS' means to add the provided flags;
and '-FLAGS' means to remove them.  +flags+ is a list of flags.

The return value is an array of Net::IMAP::FetchData. For example:

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]}>]