def stack
      s = Thread.current.thread_variable_get(STACK_NAME)
      if s.nil?
        s = [{}]
        Thread.current.thread_variable_set(STACK_NAME, s)
      end
      return s
    end
def stack
      s = Thread.current.thread_variable_get(STACK_NAME)
      if s.nil?
        s = [{}]
        Thread.current.thread_variable_set(STACK_NAME, s)
      end
      return s
    end
Returns the stack of Hash objects that are storing the diagnostic
 context information. This stack is guarnteed to always contain at least
 one Hash.