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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 notebook_type(self, notebook_type):
        ''' Notebook type, acceptable values are 'jupyter' as well as any names
        defined by external notebook hooks that have been installed.

        '''
        if notebook_type is None or not isinstance(notebook_type, string_types):
            raise ValueError("Notebook type must be a string")
        self._notebook_type = notebook_type.lower()