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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 RegisterPlugin(cls, plugin_class):
 """Registers a plugin class. The plugin classes are identified based on their lower case name. Args: plugin_class (type):
 class of the plugin. Raises: KeyError: if plugin class is already set for the corresponding name. """

    plugin_name = plugin_class.NAME.lower()
    if plugin_name in cls._plugin_classes:
      raise KeyError((
          'Plugin class already set for name: {0:s}.').format(
              plugin_class.NAME))

    cls._plugin_classes[plugin_name] = plugin_class