anchor.diff:  It takes a `Function` that maps the error to the fallback sequence to switch to:
   [source,java]
   ----
  -Flux.just("key1", "key2")
  +Flux.just("timeout1", "unknown", "key2")
       .flatMap(k ->
           callExternalService(k)
             .onErrorResumeWith(error -> { <1>
  
positive.code: ----
  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:
  [source,java]
  ----
  Flux.just(10)
      .map(this::doSomethingDangerous)
      .onErrorReturn(e -> valueForError(e));
  ----
  
  If you want more than a single default value and you have an alternative safer
  
negative.code: wrap and rethrow) could be achieved:
  [source,java]
  ----
  Flux.just("timeout1")
      .flatMap(k -> callExternalService(k)
          .onErrorResumeWith(original -> Flux.error(
              new BusinessException("oops, SLA exceeded", original))
          )
      );
  ----
  