src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable compose(CompletableTransformer transformer) {
        return wrap(ObjectHelper.requireNonNull(transformer, "transformer is null").apply(this));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable compose(CompletableTransformer transformer) {
        return wrap(ObjectHelper.requireNonNull(transformer, "transformer is null").apply(this));
    }
Calls the given transformer function with this instance and returns the function's resulting
Completable.
<p>
<img width="640" height="625" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.compose.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code compose} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param transformer the transformer function, not null
@return the Completable returned by the function
@throws NullPointerException if transformer is null