Critical behavior and restoration. (a) 2-variable ND solutions with perturbations to avoid converging to zero after a critical perturbation. |$x_1$| and |$x_2$| come back to life after the second perturbation and (b) Conservation law Ψ with the two perturbations in (a). Ψ recovers from the perturbation after Sp. 2–Ep. 2.