template<typename Sample, typename Weight, typename MeanFeature> 
struct lazy_weighted_variance_impl {
  // types
  typedef numeric::functional::multiplies< Sample, Weight >::result_type       weighted_sample;
  typedef numeric::functional::average< weighted_sample, Weight >::result_type result_type;    

  // construct/copy/destruct
  lazy_weighted_variance_impl(dont_care);

  // public member functions
  template<typename Args> result_type result(Args const &) const;
};