Apply SNoLev_ind2 with
λ x0 x1 . add_SNo x0 x1 = add_SNo x1 x0.
Let x0 of type ι be given.
Let x1 of type ι be given.
Apply add_SNo_eq with
x0,
x1,
λ x2 x3 . x3 = add_SNo x1 x0 leaving 3 subgoals.
The subproof is completed by applying H0.
The subproof is completed by applying H1.