code: public final Flux<T> expand(Function<? super T, ? extends Publisher<? extends T>> expander,
  			int capacityHint) {
  		return Flux.onAssembly(new MonoExpand<>(this, expander, true, capacityHint));
  	}
docs: Recursively expand elements into a graph and emit all the resulting element using
  a breadth-first traversal strategy.
  <p>
  That is: emit the value from this {@link Mono} first, then it 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
  - AB
  - ab1
  - a1
  </pre>
  
  Expands {@code Mono.just(A)} into
  <pre>
  A
  AA
  AB
  a1
  aa1
  ab1
  </pre>
  
  @param expander the {@link Function} applied at each level of recursion to expand
  values into a {@link Publisher}, producing a graph.
  @param capacityHint a capacity hint to prepare the inner queues to accommodate n
  elements per level of recursion.
  
  @return this Mono expanded breadth-first to a {@link Flux}