def sub_inspect
      s = ""
      if !@object.nil?
        s += format("%x ", @object.object_id)
      end
      s + "{" + keys.collect { |k| "#{k} => #{self[k].sub_inspect}" }.join(",") + "}"
    end
def sub_inspect
      s = ""
      if !@object.nil?
        s += format("%x ", @object.object_id)
      end
      s + "{" + keys.collect { |k| "#{k} => #{self[k].sub_inspect}" }.join(",") + "}"
    end
Return instance inspect information, used by Node#inspect.