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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 pst(self):
 """ get the pyemu.Pst attribute Returns ------- pst : pyemu.Pst Note ---- returns a references If LinearAnalysis.__pst is None, then the pst attribute is dynamically loaded before returning """

        if self.__pst is None and self.pst_arg is None:
            raise Exception("linear_analysis.pst: can't access self.pst:" +
                            "no pest control argument passed")
        elif self.__pst:
            return self.__pst
        else:
            self.__load_pst()
            return self.__pst