template<typename SortPolicy,
         template<typename TreeMetricType,
                  typename TreeStatType,
                  typename TreeMatType> class TreeType>
class NSTypeClass
{
public:
  using NSType = NeighborSearch<SortPolicy,
  metric::EuclideanDistance,
  arma::mat,
  TreeType,
  TreeType<metric::EuclideanDistance,
      NeighborSearchStat<SortPolicy>,
      arma::mat>::template DualTreeTraverser>;
};

class TrainVisitor : public boost::static_visitor<void>
{
 private:
   arma::mat&& referenceSet;
   size_t leafSize;

   template<typename NSType>
   void TrainLeaf(NSType* ns) const;

 public:
   //tree::KDTree
   template<typename SortPolicy>
   void operator()(typename NSTypeClass<SortPolicy,tree::KDTree>::NSType *ns) const
   {
     if (ns)
       return TrainLeaf(ns);
     throw std::runtime_error("no neighbor search model initialized");
   }

   //tree::BallTree
   template<typename SortPolicy>
   void operator()(typename NSTypeClass<SortPolicy,tree::BallTree>::NSType *ns) const
   {
     if (ns)
       return TrainLeaf(ns);
     throw std::runtime_error("no neighbor search model initialized");
   }//*/

   template<typename NSType>
   void operator()(NSType *ns) const
   {
     if (ns)
       return ns->Train(std::move(referenceSet));
     throw std::runtime_error("no neighbor search model initialized");
   }

   TrainVisitor(arma::mat&& referenceSet, const size_t leafSize);
};