/// Lognormal distribution

     /// Lognormal distribution. The parameters are the mean and the standard

     /// deviation of <tt>exp(X)</tt>.

     ///

     double lognormal(double n_mean,double n_std_dev)

     {

       return std::exp(gauss(n_mean,n_std_dev));

     }

     /// Lognormal distribution

     /// Lognormal distribution. The parameter is an <tt>std::pair</tt> of

     /// the mean and the standard deviation of <tt>exp(X)</tt>.

     ///

     double lognormal(const std::pair<double,double> &params)

     {

       return std::exp(gauss(params.first,params.second));

     }

     /// Compute the lognormal parameters from mean and standard deviation

     /// This function computes the lognormal parameters from mean and

     /// standard deviation. The return value can direcly be passed to

     /// lognormal().

     std::pair<double,double> lognormalParamsFromMD(double mean,

 						   double std_dev)

     {

       double fr=std_dev/mean;

       fr*=fr;

       double lg=std::log(1+fr);

       return std::pair<double,double>(std::log(mean)-lg/2.0,std::sqrt(lg));

     }

     /// Lognormal distribution with given mean and standard deviation

     /// Lognormal distribution with given mean and standard deviation.

     ///

     double lognormalMD(double mean,double std_dev)

     {

       return lognormal(lognormalParamsFromMD(mean,std_dev));

     }