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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 get_atom_type_symbol(cls,calc,**kwargs):
 """ Returns a list of atom types. Each atom site MUST occur only once in this list. List MUST be sorted. """

        parameters = calc.out.output
        dictionary = parameters.get_dict()
        if 'basis_set' not in dictionary.keys():
            return None
        return sorted(dictionary['basis_set'].keys())