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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 AddClass(self, class_name, gtfs_class):
 """Adds an entry to the list of known classes. Args: class_name: A string with name through which gtfs_class is to be made accessible. gtfs_class: The class to be added. Raises: DuplicateMapping if class_name is already present in the class mapping. """

    if class_name in self._class_mapping:
      raise problems.DuplicateMapping(class_name)
    self._class_mapping[class_name] = gtfs_class