std::list<intrusive_ptr<DocumentSource>> DocumentSourceExtensionDesugar::getDesugaredPipeline() {
// returns a copy of the lazy initialized (i.e Deferred) desugared Pipeline
}

// Desugarer accepts a Pipeline* in its constructor, which is the Pipeline that needs to be desugared. 
class Desugarer {
public:

Desugarer(Pipeline* pipeline) : m_pipeline (pipeline), m_sources(pipeline->getSources) {}

void operator() {
   try {
      auto itr = m_sources.begin();
      while (itr != m_sources.end()) {
          invariant(itr.get());
          itr = _desugar(itr, *itr.get());
      }
   } catch (...) {
     
   }
}

DocumentSourceContainer::iterator _desugar(DocumentSourceContainer::iterator, DocumentSource& stage) {
   invariant(itr->get() == &stage);
   // Non-Desugar stages just advance to the next stage in the pipeline.
   return std::next(itr);
}


DocumentSourceContainer::iterator _desugar(DocumentSourceContainer::iterator, DocumentSourceExtensionDesugar& stage) {
    invariant(itr->get() == &stage);
   
    auto desugaredPipeline = stage.getDesugaredPipeline();
    // replace desugar stage with its desugared pipeline
    if (!desugaredPipeline.empty()) {
       auto desugarIterator = itr;
       m_sources.insert(itr, desugaredPipeline.begin(), desugaredPipeline.end());
       itr = std::next(itr);
       m_sources.erase(desugarIterator);
    }

    return itr; 
}

private:
Pipeline* m_pipeline;
DocumentSourceContainer& m_sources;
}