mutex_m.rb -
    $Release Version: 3.0$
    $Revision: 1.7 $
    Original from mutex.rb
    by Keiju ISHITSUKA(keiju@ishitsuka.com)
    modified by matz
    patched by akira yamada

Usage:
    require "mutex_m.rb"
    obj = Object.new
    obj.extend Mutex_m
    ...
    extended object can be handled like Mutex
    or
    class Foo
      include Mutex_m
      ...
    end
    obj = Foo.new
    this obj can be handled like Mutex