reactor-core/src/main/java/reactor/core/publisher/ParallelFlux.java
public final <U> ParallelFlux<U> map(Function<? super T, ? extends U> mapper) {
		Objects.requireNonNull(mapper, "mapper");
		return onAssembly(new ParallelMap<>(this, mapper));
	}
public final <U> ParallelFlux<U> map(Function<? super T, ? extends U> mapper) {
		Objects.requireNonNull(mapper, "mapper");
		return onAssembly(new ParallelMap<>(this, mapper));
	}
Maps the source values on each 'rail' to another value.
<p>
Note that the same mapper function may be called from multiple threads
concurrently.

@param <U> the output value type
@param mapper the mapper function turning Ts into Us.

@return the new {@link ParallelFlux} instance