A 
poisson_distribution random number distribution
produces integer values 
i≥0
distributed according to
the discrete probability function
P(i|μ)=e−μμii! .  
The distribution parameter 
μ
is also known as this distribution's 
mean.template<class IntType = int>
  class poisson_distribution
  {
  public:
    
    using result_type = IntType;
    using param_type  = unspecified;
    
    poisson_distribution() : poisson_distribution(1.0) {}
    explicit poisson_distribution(double mean);
    explicit poisson_distribution(const param_type& parm);
    void reset();
    
    template<class URBG>
      result_type operator()(URBG& g);
    template<class URBG>
      result_type operator()(URBG& g, const param_type& parm);
    
    double mean() const;
    param_type param() const;
    void param(const param_type& parm);
    result_type min() const;
    result_type max() const;
  };