<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def total_variation(domain, grad=None):
 """Total variation functional. Parameters domain : odlspace domain of TV functional grad : gradient operator, optional Gradient operator of the total variation functional. This may be any linear operator and thereby generalizing TV. default=forward differences with Neumann boundary conditions Examples -------- Check that the total variation of a constant is zero """


    if grad is None:
        grad = odl.Gradient(domain, method='forward', pad_mode='symmetric')
        grad.norm = 2 * np.sqrt(sum(1 / grad.domain.cell_sides**2))
    else:
        grad = grad

    f = odl.solvers.GroupL1Norm(grad.range, exponent=2)

    return f * grad