reactor-core/src/main/java/reactor/core/publisher/Flux.java
public final <V> Flux<V> concatMapDelayError(Function<? super T, ? extends Publisher<? extends V>> mapper) {
		return concatMapDelayError(mapper, Queues.XS_BUFFER_SIZE);
	}
public final <V> Flux<V> concatMapDelayError(Function<? super T, ? extends Publisher<? extends V>> mapper) {
		return concatMapDelayError(mapper, Queues.XS_BUFFER_SIZE);
	}
Transform the elements emitted by this {@link Flux} asynchronously into Publishers,
then flatten these inner publishers into a single {@link Flux}, sequentially and
preserving order using concatenation.
<p>
There are three dimensions to this operator that can be compared with
{@link #flatMap(Function) flatMap} and {@link #flatMapSequential(Function) flatMapSequential}:
<ul>
<li><b>Generation of inners and subscription</b>: this operator waits for one
inner to complete before generating the next one and subscribing to it.</li>
<li><b>Ordering of the flattened values</b>: this operator naturally preserves
the same order as the source elements, concatenating the inners from each source
element sequentially.</li>
<li><b>Interleaving</b>: this operator does not let values from different inners
interleave (concatenation).</li>
</ul>

<p>
Errors in the individual publishers will be delayed at the end of the whole concat
sequence (possibly getting combined into a {@link Exceptions#isMultiple(Throwable) composite}
if several sources error.

<p>
<img class="marble" src="doc-files/marbles/concatMap.svg" alt="">

@reactor.discard This operator discards elements it internally queued for backpressure upon cancellation.

@param mapper the function to transform this sequence of T into concatenated sequences of V
@param <V> the produced concatenated type

@return a concatenated {@link Flux}