Apply SNoLev_ind with
λ x0 . add_SNo (minus_SNo x0) x0 = 0.
Let x0 of type ι be given.
Apply add_SNo_eq with
minus_SNo x0,
x0,
λ x1 x2 . x2 = 0 leaving 3 subgoals.
The subproof is completed by applying L2.
The subproof is completed by applying H0.