--- first
--- first(n)    ruby 1.8 feature

    ƬǤ֤ޤǤʤ nil ֤ޤ

    :

        p [0, 1, 2].first   #=> 0
        p [].first          #=> nil

    ruby 1.8 feature:
    άǽʰ n ꤷ硢Ƭ n Ǥ
    ֤ޤn  0 ʾǤʤФʤޤ

        ary =  [0, 1, 2]
        p ary.first(0)
        p ary.first(1)
        p ary.first(2)
        p ary.first(3)
        p ary.first(4)
        # => []
             [0]
             [0, 1]
             [0, 1, 2]
             [0, 1, 2]

    Array#last [Array/last] ⻲ȤƤ

