--- flatten!
#    Equivalent to Set#flatten, but replaces the receiver with the
#    result in place.  Returns nil if no modifications were made.
    flatten ƱǤ
    礽켫Ȥʿ경ƺ줿Ǥ֤ޤ
    ǤѹʤäȤˤ nil ֤ޤ

