src/main/java/io/reactivex/Maybe.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> toSingle() {
        return RxJavaPlugins.onAssembly(new MaybeToSingle<T>(this, null));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> toSingle() {
        return RxJavaPlugins.onAssembly(new MaybeToSingle<T>(this, null));
    }
Converts this Maybe into a Single instance composing disposal
through and turning an empty Maybe into a signal of NoSuchElementException.
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code toSingle} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@return the new Single instance