code: def fitness(self, parsimony_coefficient=None):
          
          if parsimony_coefficient is None:
              parsimony_coefficient = self.parsimony_coefficient
          penalty = parsimony_coefficient * len(self.program) * self.metric.sign
          return self.raw_fitness_ - penalty
docs: Evaluate the penalized fitness of the program according to X, y.
  
          Parameters
          ----------
          parsimony_coefficient : float, optional
              If automatic parsimony is being used, the computed value according
              to the population. Otherwise the initialized value is used.
  
          Returns
          -------
          fitness : float
              The penalized fitness of the program.