def capability
      synchronize do
        send_command("CAPABILITY")
        return @responses.delete("CAPABILITY")[-1]
      end
    end
Sends a CAPABILITY command, and returns an array of
capabilities that the server supports.  Each capability
is a string.  See [IMAP] for a list of possible
capabilities.

Note that the Net::IMAP class does not modify its
behaviour according to the capabilities of the server;
it is up to the user of the class to ensure that
a certain capability is supported by a server before
using it.