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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 UpdateClass(self, class_name, gtfs_class):
 """Updates an entry in the list of known classes. Args: class_name: A string with the class name that is to be updated. gtfs_class: The new class Raises: NonexistentMapping if there is no class with the specified class_name. """

    if class_name not in self._class_mapping:
      raise problems.NonexistentMapping(class_name)
    self._class_mapping[class_name] = gtfs_class