code: def compute_quantity(self, quantity):
          
          quantity = quantity.copy()
          expression = self.model.replace(quantity.pop('expression'), self.replacements)
          variables = self.all_variables[len(self.free_variables) + 1:]
          function = self.model.lambdify(expression, variables, **self.lambdify_options)
          quantity['value'] = function(*(self.fitted_parameters + self.fixed_values)) * prefix_factor(quantity)**(-1)
          return quantity
docs: Processes a single quantity as described and used
          in `scipy_data_fitting.Fit.computed_quantities`.