anchor.diff:      // Exercise system
       sut.Customize(fixtureStub)
       // Verify outcome
  -    Assert.Contains(sut.Relay, residueCollectors)
  +    verify <@ Seq.exists ((=) sut.Relay) <| residueCollectors @>
       // Teardown
   
   [<Fact>]
  
positive.code:     // Exercise system
      let result = sut.Relay
      // Verify outcome
      Assert.Equal(expectedBuilder, result)
      // Teardown
  
  [<Fact>]
  
negative.code:     // Exercise system
      sut.Customize(fixtureStub)
      // Verify outcome
      verify <@ customizations |> Seq.isEmpty @>
      // Teardown
  
  