// In header: <boost/accumulators/statistics/covariance.hpp>
template<typename Left, typename Right> 
struct outer_product<Left, Right, std_vector_tag, std_vector_tag> : public std::binary_function< Left, Right, ublas::matrix< functional::multiplies< Left::value_type, Right::value_type >::result_type > >
{
  // types
  typedef ublas::matrix< typename functional::multiplies< typename Left::value_type, typename Right::value_type >::result_type > result_type;
  // public member functions
  result_type operator()(Left &, Right &) const;
};