src/main/java/io/reactivex/internal/operators/observable/ObservableScalarXMap.java
public static <T, U> Observable<U> scalarXMap(T value,
            Function<? super T, ? extends ObservableSource<? extends U>> mapper) {
        return RxJavaPlugins.onAssembly(new ScalarXMapObservable<T, U>(value, mapper));
    }
public static <T, U> Observable<U> scalarXMap(T value,
            Function<? super T, ? extends ObservableSource<? extends U>> mapper) {
        return RxJavaPlugins.onAssembly(new ScalarXMapObservable<T, U>(value, mapper));
    }
Maps a scalar value into an Observable 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
an ObservableSource that gets streamed
@return the new Observable instance