global

def around(*args, &block)
  raise ArgumentError, "only :each or no argument is supported" if args != [] && args != [:each]
  fib = nil
  before do
    fib = Fiber.new do |context, resume|
      begin
        context.instance_exec(resume, &block)
      rescue Object
        fib = :failed
        raise
      end
    end
    fib.resume(self, lambda { Fiber.yield })
  end
  after  { fib.resume if fib && fib != :failed }
end