code: public final Flux<T> expandDeep(Function<? super T, ? extends Publisher<? extends T>> expander) {
  		return expandDeep(expander, Queues.SMALL_BUFFER_SIZE);
  	}
docs: Recursively expand elements into a graph and emit all the resulting element,
  in a depth-first traversal order.
  <p>
  That is: emit one value from this {@link Flux}, expand it and emit the first value
  at this first level of recursion, and so on... When no more recursion is possible,
  backtrack to the previous level and re-apply the strategy.
  <p>
  For example, given the hierarchical structure
  <pre>
  A
  - AA
  - aa1
  B
  - BB
  - bb1
  </pre>
  
  Expands {@code Flux.just(A, B)} into
  <pre>
  A
  AA
  aa1
  B
  BB
  bb1
  </pre>
  
  @param expander the {@link Function} applied at each level of recursion to expand
  values into a {@link Publisher}, producing a graph.
  
  @return a {@link Flux} expanded depth-first