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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def write_concrete_dag(self):
 """ Write all the nodes in the DAG to the DAG file. """

    if not self.__dag_file_path:
      raise CondorDAGError, "No path for DAG file"
    try:
      dagfile = open( self.__dag_file_path, 'w' )
    except:
      raise CondorDAGError, "Cannot open file " + self.__dag_file_path
    for node in self.__nodes:
      node.write_job(dagfile)
      node.write_vars(dagfile)
      if node.get_category():
        node.write_category(dagfile)
      if node.get_priority():
        node.write_priority(dagfile)
      node.write_pre_script(dagfile)
      node.write_post_script(dagfile)
      node.write_input_files(dagfile)
      node.write_output_files(dagfile)
    for node in self.__nodes:
      node.write_parents(dagfile)
    for category in self.__maxjobs_categories:
      self.write_maxjobs(dagfile, category)
    dagfile.close()