--- default_proc                ruby 1.7 feature

    ϥåΥǥե֤ͤ Proc ֥Ȥ֤ޤ
    ǥեȤ Proc ʤ nil ֤ޤ

        h = Hash.new {|h, k| }
        p h.default_proc        # => #<Proc:0x0x401a9ff4>

        h = Hash.new(1)
        p h.default_proc        # => nil

