code: def _set_err_paramvals(self):
          
          # self.param_vals = p0 #sloppy...
          self._last_vals = self.param_vals.copy()
          self.error = self.update_function(self.param_vals)
          self._last_error = (1 + 2*self.fractol) * self.error
docs: Must update:
              self.error, self._last_error, self.param_vals, self._last_vals