function it_processes_etl_sequentially(ExtractorInterface $extractor, TransformerInterface $transformer, LoaderInterface $loader)
 {
     $extractor->extract()->shouldBeCalled()->willReturn(['a1', 'b1', 'c1'], null);
     $transformer->transform(['a1', 'b1', 'c1'])->shouldBeCalled()->willReturn(['c1', 'b1', 'a1']);
     $loader->load(['c1', 'b1', 'a1'])->shouldBeCalled();
     $loader->flush()->shouldBeCalled();
     $this->process();
 }