code: def add_independent_variable(self, variable):
          
  
          assert isinstance(variable, IndependentVariable), "Variable must be an instance of IndependentVariable"
  
          if self._has_child(variable.name):
  
              self._remove_child(variable.name)
  
          self._add_child(variable)
  
          # Add also to the list of independent variables
          self._independent_variables[variable.name] = variable
docs: Add a global independent variable to this model, such as time.
  
          :param variable: an IndependentVariable instance
          :return: none