code: def all_variables(self):
          
          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)
docs: 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`