def METHOD_NAME(ts, coors, bc, problem):
    t = ts.time
    x_1, x_2 = coors[..., 0], coors[..., 1]
    res = nm.zeros(x_1.shape)
    if bc.diff == 0:
        res[:] = analytic_sol(coors, t)
    elif bc.diff == 1:
        res = nm.stack((x_1 - a, -x_2 + b),
                       axis=-2)
    return res