src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final <U> U to(@NonNull Function<? super ParallelFlowable<T>, U> converter) {
        try {
            return ObjectHelper.requireNonNull(converter, "converter is null").apply(this);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            throw ExceptionHelper.wrapOrThrow(ex);
        }
    }
@CheckReturnValue
    @NonNull
    public final <U> U to(@NonNull Function<? super ParallelFlowable<T>, U> converter) {
        try {
            return ObjectHelper.requireNonNull(converter, "converter is null").apply(this);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            throw ExceptionHelper.wrapOrThrow(ex);
        }
    }
Perform a fluent transformation to a value via a converter function which
receives this ParallelFlowable.

@param <U> the output value type
@param converter the converter function from ParallelFlowable to some type
@return the value returned by the converter function