--- assoc(key)

    򸡺ơ 0 ܤǤ key ǽ
    Ǥ֤ޤӤ == 黻ҤȤäƹԤޤ
    Ǥʤ nil ֤ޤ

    :
        ary = [[1,15], [2,25], [3,35]]
        p ary.assoc(2)           # => [2, 25]
        p ary.assoc(100)         # => nil
        p ary.assoc(15)          # => nil

    Array#rassoc [Array/rassoc] ⻲ȤƤ

