src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> U to(Function<? super Completable, U> converter) {
        try {
            return ObjectHelper.requireNonNull(converter, "converter is null").apply(this);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            throw ExceptionHelper.wrapOrThrow(ex);
        }
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> U to(Function<? super Completable, U> converter) {
        try {
            return ObjectHelper.requireNonNull(converter, "converter is null").apply(this);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            throw ExceptionHelper.wrapOrThrow(ex);
        }
    }
Allows fluent conversion to another type via a function callback.
<p>
<img width="640" height="751" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.to.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code to} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <U> the output type
@param converter the function called with this which should return some other value.
@return the converted value
@throws NullPointerException if converter is null