--- enclose             ruby 1.8 feature

  ThreadGroup ؤ Thread ɲ/(freeze )ػߤޤ
  (ɲ/Ԥ㳰 ThreadError ȯޤ)

  self ֤ޤ

  ɲä:

        thg = ThreadGroup.new.enclose
        thg.add Thread.new {}

        => -:2:in `add': can't move to the enclosed thread group (ThreadError)

  :

        thg1 = ThreadGroup.new
        thg2 = ThreadGroup.new

        th = Thread.new {sleep 1}

        thg1.add th
        thg1.enclose
        thg2.add th

        => -:8:in `add': can't move from the enclosed thread group (ThreadError)

