private protected override IEnumerable<IList<IBar>> GenerateMelody()
       {
           // get first generatiion 
           PopulateFirstGeneration();

           int i = 0;
           bool terminateCondition = false;

           while (!terminateCondition)
           {
               // update generation counter 
               _currentGeneration++;

               // mix and combine pieces of different individuals 
               Crossover();

               // modify parts of individuals 
               Mutate();

               // rate each individual 
               EvaluateFitness();

               // natural selection 
               SelectNextGeneration();

               // Check if termination conditions are met 
               if (++i == MaxNumberOfIterations || (_candidates.Select(c => c.FitnessGrade).Max() >= CuttingEvaluationGrade))
                       terminateCondition = true;                   
           }
           //...
           // return the result 
           IEnumerable<IList<IBar>> composedMelodies = _candidates
               .OrderByDescending(c => c.FitnessGrade)
               .Select(c => c.Bars);
           return composedMelodies;
      }