code: def add_absolute_expression(model, expression, name="abs_var", ub=None,
                              difference=0, add=True):
      
      Components = namedtuple('Components', ['variable', 'upper_constraint',
                                             'lower_constraint'])
      variable = model.problem.Variable(name, lb=0, ub=ub)
      # The following constraints enforce variable > expression and
      # variable > -expression
      upper_constraint = model.problem.Constraint(expression - variable,
                                                  ub=difference,
                                                  name="abs_pos_" + name),
      lower_constraint = model.problem.Constraint(expression + variable,
                                                  lb=difference,
                                                  name="abs_neg_" + name)
      to_add = Components(variable, upper_constraint, lower_constraint)
      if add:
          add_cons_vars_to_problem(model, to_add)
      return to_add
docs: Add the absolute value of an expression to the model.
  
      Also defines a variable for the absolute value that can be used in other
      objectives or constraints.
  
      Parameters
      ----------
      model : a cobra model
         The model to which to add the absolute expression.
      expression : A sympy expression
         Must be a valid expression within the Model's solver object. The
         absolute value is applied automatically on the expression.
      name : string
         The name of the newly created variable.
      ub : positive float
         The upper bound for the variable.
      difference : positive float
          The difference between the expression and the variable.
      add : bool
          Whether to add the variable to the model at once.
  
      Returns
      -------
      namedtuple
          A named tuple with variable and two constraints (upper_constraint,
          lower_constraint) describing the new variable and the constraints
          that assign the absolute value of the expression to it.