old_contents: class RungeKuttaEvolve(object):
      def __init__(self, alpha, gamma_G=2.210173e5,
                   start_dm=0.01, method='rkf54'):
          if not isinstance(alpha, (int, float)) or alpha < 0:
              raise ValueError('alpha must be a positive float or int.')
          else:
              self.alpha = alpha
  
          if not isinstance(gamma_G, (float, int)) or gamma_G <= 0:
              raise ValueError('gamma_G must be a positive float or int.')
          else:
              self.gamma_G = gamma_G
  
          if not isinstance(start_dm, (float, int)) or start_dm <= 0:
              raise ValueError('start_dm must be a positive float or int.')
          else:
              self.start_dm = start_dm
  
          if not isinstance(method, str):
              raise ValueError('solver must be a string')
          else:
              self.method = method
  
      def get_mif(self):
          # Create mif string.
          mif = '# RungeKutta evolver\n'
          mif += 'Specify Oxs_RungeKuttaEvolve:evolver {\n'
          mif += '\talpha {}\n'.format(self.alpha)
          mif += '\tgamma_G {}\n'.format(self.gamma_G)
          mif += '\tstart_dm {}\n'.format(self.start_dm)
          mif += '\tmethod {}\n'.format(self.method)
          mif += '}\n\n'
  
          return mif
  
new_contents: class RungeKuttaEvolve(object):
      def __init__(self, alpha, gamma_G=2.210173e5,
                   start_dm=0.01, method='rkf54'):
          if not isinstance(alpha, (int, float)) or alpha < 0:
              raise ValueError('alpha must be a positive float or int.')
          else:
              self.alpha = alpha
  
          if not isinstance(gamma_G, (float, int)) or gamma_G <= 0:
              raise ValueError('gamma_G must be a positive float or int.')
          else:
              self.gamma_G = gamma_G
  
          if not isinstance(start_dm, (float, int)) or start_dm <= 0:
              raise ValueError('start_dm must be a positive float or int.')
          else:
              self.start_dm = start_dm
  
          if not isinstance(method, str):
              raise ValueError('solver must be a string')
          else:
              self.method = method
  
      def get_mif(self):
          # Create mif string.
          mif = '# RungeKutta evolver\n'
          mif += 'Specify Oxs_RungeKuttaEvolve:evolver {\n'
          mif += '\talpha {}\n'.format(self.alpha)
          mif += '\tgamma_G {}\n'.format(self.gamma_G)
          mif += '\tstart_dm {}\n'.format(self.start_dm)
          mif += '\tmethod {}\n'.format(self.method)
          mif += '}\n\n'
  
          return mif
  
  
  class CGEvolve(object):
      def __init__(self):
          pass
  
      def get_mif(self):
          mif = '# CG evolver\n'
          mif += 'Specify Oxs_CGEvolve {}\n\n'
  
          return mif
  
subject: Add CGEvolver.