anchor.diff:  ----
   Flux.just(10)
       .map(this::doSomethingDangerous)
  -    .onErrorReturn("DEFAULT_VALUE");
  -----
  -
  -You also have the option of dynamically choosing the single value depending on
  -the exception that occurred:
  +    .onErrorReturn("RECOVERED");
  +----
  +
  +You also have the option of filtering when to recover with a default value vs
  +letting the error propagate, depending on the exception that occurred:
   [source,java]
   ----
   Flux.just(10)
       .map(this::doSomethingDangerous)
  -    .onErrorReturn(e -> valueForError(e));
  +    .onErrorReturn(e -> e.getMessage().equals("boom10"), "recovered10");
   ----
   
   If you want more than a single default value and you have an alternative safer
  
positive.code: we have statistic counters to increment as an error side-effect...
  [source,java]
  ----
  Flux.just("key1")
      .flatMap(k -> callExternalService(k) <1>
          .doOnError(e -> log("uh oh, falling back, service failed for key " + k)) <2>
          .switchOnError(getFromCache(k)) <3>
      );
  ----
  <1> the external service call that can fail...
  
negative.code: It takes a `Function` that maps the error to the fallback sequence to switch to:
  [source,java]
  ----
  Flux.just("timeout1", "unknown", "key2")
      .flatMap(k ->
          callExternalService(k)
            .onErrorResumeWith(error -> { <1>
  