code: def serialize_model(model):
      
  
      result = ''
  
      # First serialize the macros
      for macro in model['macros']:
          if macro.get('options', None):
              result += '~o '
              for option in macro['options']['definition']:
                  result += _serialize_option(option)
  
          elif macro.get('transition', None):
              result += '~t "{}"\n'.format(macro['transition']['name'])
              result += _serialize_transp(macro['transition']['definition'])
  
          elif macro.get('variance', None):
              result += '~v "{}"\n'.format(macro['variance']['name'])
              result += _serialize_variance(macro['variance']['definition'])
  
          elif macro.get('state', None):
              result += '~s "{}"\n'.format(macro['state']['name'])
              result += _serialize_stateinfo(macro['state']['definition'])
  
          elif macro.get('mean', None):
              result += '~u "{}"\n'.format(macro['mean']['name'])
              result += _serialize_mean(macro['mean']['definition'])
  
          elif macro.get('duration', None):
              result += '~d "{}"\n'.format(macro['duration']['name'])
              result += _serialize_duration(macro['duration']['definition'])
  
          else:
              raise NotImplementedError('Cannot serialize {}'.format(macro))
  
      for hmm in model['hmms']:
          if hmm.get('name', None) is not None:
              result += '~h "{}"\n'.format(hmm['name'])
  
          result += _serialize_hmm(hmm['definition'])
  
      return result
docs: Serialize the HTK model into a file.
  
      :param model: Model to be serialized