A flat tuple of all symbols taken in order from the following:

        1. `scipy_data_fitting.Fit.free_variables`
        2. `scipy_data_fitting.Fit.independent`
        3. `scipy_data_fitting.Fit.fitting_parameters`
        4. `scipy_data_fitting.Fit.fixed_parameters`
        5. `scipy_data_fitting.Fit.constants`

def all_variables(self):
        """
        A flat tuple of all symbols taken in order from the following:

        1. `scipy_data_fitting.Fit.free_variables`
        2. `scipy_data_fitting.Fit.independent`
        3. `scipy_data_fitting.Fit.fitting_parameters`
        4. `scipy_data_fitting.Fit.fixed_parameters`
        5. `scipy_data_fitting.Fit.constants`
        """
        variables = []
        variables.extend(self.free_variables)
        variables.append(self.independent['symbol'])
        variables.extend([ param['symbol'] for param in self.fitting_parameters ])
        variables.extend([ param['symbol'] for param in self.fixed_parameters ])
        variables.extend([ const['symbol'] for const in self.constants ])

        symbols = []
        for variable in variables:
            if isinstance(variable, str):
                symbols.append(self.model.symbol(variable))
            else:
                symbols.append(variable)

        return tuple(symbols)