code: public final Flux<T> expand(Function<? super T, ? extends Publisher<? extends T>> expander) {
  		return expand(expander, Queues.SMALL_BUFFER_SIZE);
  	}
docs: Recursively expand elements into a graph and emit all the resulting element using
  a breadth-first traversal strategy.
  <p>
  That is: emit the values from this {@link Flux} first, then expand each at a first level of
  recursion and emit all of the resulting values, then expand all of these at a second
  level and so on..
  <p>
  For example, given the hierarchical structure
  <pre>
  A
  - AA
  - aa1
  B
  - BB
  - bb1
  </pre>
  
  Expands {@code Flux.just(A, B)} into
  <pre>
  A
  B
  AA
  BB
  aa1
  bb1
  </pre>
  
  @param expander the {@link Function} applied at each level of recursion to expand
  values into a {@link Publisher}, producing a graph.
  
  @return an breadth-first expanded {@link Flux}