anchor.diff:  `Flux<Void>`/`Mono<Void>` sources, as you can only switch to another `Publisher<Void>`,
   which is still guaranteed to be empty. The following example uses `defaultIfEmpty`:
   
  -====
  +
   .use `defaultIfEmpty` before `zipWhen`
   ====
   [source,java]
  
positive.code: 
  The following sample is even better (because it is simpler):
  
  ====
  .with chaining (best)
  ====
  [source,java]
  
negative.code: 
  Compare the following two examples:
  
  
  .without chaining (incorrect)
  ====
  [source,java]
  