code: def pst(self):
          
          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
docs: 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