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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 function(self):
 """ The function passed to the `fit_function` specified in `scipy_data_fitting.Fit.options`, and used by `scipy_data_fitting.Fit.pointspace` to generate plots, etc. Its number of arguments and their order is determined by items 1, 2, and 3 as listed in `scipy_data_fitting.Fit.all_variables`. All parameter values will be multiplied by their corresponding prefix before being passed to this function. By default, it is a functional form of `scipy_data_fitting.Fit.expression` converted using `scipy_data_fitting.Model.lambdify`. See also `scipy_data_fitting.Fit.lambdify_options`. """

        if not hasattr(self,'_function'):
            function = self.model.lambdify(self.expression, self.all_variables, **self.lambdify_options)
            self._function = lambda *x: function(*(x + self.fixed_values))
        return self._function