src/main/java/io/reactivex/internal/operators/flowable/FlowableScalarXMap.java
public static <T, U> Flowable<U> scalarXMap(final T value, final Function<? super T, ? extends Publisher<? extends U>> mapper) {
        return RxJavaPlugins.onAssembly(new ScalarXMapFlowable<T, U>(value, mapper));
    }
public static <T, U> Flowable<U> scalarXMap(final T value, final Function<? super T, ? extends Publisher<? extends U>> mapper) {
        return RxJavaPlugins.onAssembly(new ScalarXMapFlowable<T, U>(value, mapper));
    }
Maps a scalar value into a Publisher and emits its values.

@param <T> the scalar value type
@param <U> the output value type
@param value the scalar value to map
@param mapper the function that gets the scalar value and should return
a Publisher that gets streamed
@return the new Flowable instance