--- rassoc(obj)

    self ǤȲꤷơǤǥǥå 
    1 Ǥ obj Τ򸡺ĤäǽǤ
    ޤӤ == 黻ҤȤäƹԤޤ

    Ǥʤ nil ֤ޤ

    :
        a = [[15,1], [25,2], [35,3]]
        p a.rassoc(2)    # => [25, 2]

    Array#assoc [Array/assoc] ⻲ȤƤ

