public void AssertCompleted_Multiple_Completions()
        {
            try
            {
                var to = new TestObserver <int>();

                to.OnCompleted();
                to.OnCompleted();

                to.AssertCompleted();
                Assert.Fail("Should have thrown");
            }
            catch (AssertionException)
            {
                throw;
            }
            catch (Exception ex)
            {
                var msg = ex.Message;
                Assert.True(msg.Contains("Multiple completions"));
            }
        }