/// 

      /// Cummularity.
      /// 
      /// 
      /// n          n!
      ///   C     = _____________
      ///     r      r! (n-r)!
      /// 
      /// 
      /// 
      /// 
      /// 
      private long nCr(long n,long r)
      {
         return nPr(n,n-r+1) / Factorial(r);
      }