anchor.diff:  
   [source,groovy]
   ----
  -def anySubscriber = GroovySpy(RealSubscriber, global: true)
  +RealSubscriber anySubscriber = GroovySpy(global: true)
   
   1 * new RealSubscriber("Fred")
   ----
  
positive.code: 
  [source,groovy]
  ----
  def subscriber = Spy(SubscriberImpl, constructorArgs: ["Fred"])
  ----
  
  A spy is always based on a real object. Hence you must provide a class type rather
  
negative.code: [source,groovy]
  ----
  // this is now the object under specification, not a collaborator
  MessagePersister persister = Spy {
    // stub a call on the same object
    isPersistable(_) >> true
  }
  