reactor-core/src/main/java/reactor/core/publisher/Flux.java
public final Flux<T> expandDeep(Function<? super T, ? extends Publisher<? extends T>> expander) {
		return expandDeep(expander, Queues.SMALL_BUFFER_SIZE);
	}
public final Flux<T> expandDeep(Function<? super T, ? extends Publisher<? extends T>> expander) {
		return expandDeep(expander, Queues.SMALL_BUFFER_SIZE);
	}
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