Apply nat_complete_ind with
λ x0 . Inj1 x0 = ordsucc x0.
Let x0 of type ι be given.
Assume H1:
∀ x1 . x1 ∈ x0 ⟶ Inj1 x1 = ordsucc x1.
Apply set_ext with
Inj1 x0,
ordsucc x0 leaving 2 subgoals.
Let x1 of type ι be given.
Assume H2:
x1 ∈ Inj1 x0.
Apply Inj1E with
x0,
x1,
x1 ∈ ordsucc x0 leaving 3 subgoals.
The subproof is completed by applying H2.
Assume H3: x1 = 0.
Apply H3 with
λ x2 x3 . x3 ∈ ordsucc x0.
Apply nat_0_in_ordsucc with
x0.
The subproof is completed by applying H0.
Assume H3:
∃ x2 . and (x2 ∈ x0) (x1 = Inj1 x2).
Apply exandE_i with
λ x2 . x2 ∈ x0,
λ x2 . x1 = Inj1 x2,
x1 ∈ ordsucc x0 leaving 2 subgoals.
The subproof is completed by applying H3.
Let x2 of type ι be given.
Assume H4: x2 ∈ x0.
Apply H5 with
λ x3 x4 . x4 ∈ ordsucc x0.
Apply H1 with
x2,
λ x3 x4 . x4 ∈ ordsucc x0 leaving 2 subgoals.
The subproof is completed by applying H4.
Apply nat_ordsucc_in_ordsucc with
x0,
x2 leaving 2 subgoals.
The subproof is completed by applying H0.
The subproof is completed by applying H4.
Let x1 of type ι be given.
Apply ordsuccE with
x0,
x1,
x1 ∈ Inj1 x0 leaving 3 subgoals.
The subproof is completed by applying H2.
Assume H3: x1 ∈ x0.
Apply nat_inv with
x1,
x1 ∈ Inj1 x0 leaving 3 subgoals.
Apply nat_p_trans with
x0,
x1 leaving 2 subgoals.
The subproof is completed by applying H0.
The subproof is completed by applying H3.
Assume H4: x1 = 0.
Apply H4 with
λ x2 x3 . x3 ∈ Inj1 x0.
The subproof is completed by applying Inj1I1 with x0.
Apply exandE_i with
nat_p,
λ x2 . x1 = ordsucc x2,
x1 ∈ Inj1 x0 leaving 2 subgoals.
The subproof is completed by applying H4.
Let x2 of type ι be given.
Apply H6 with
λ x3 x4 . x4 ∈ Inj1 x0.
Claim L7: x2 ∈ x1
Apply H6 with
λ x3 x4 . x2 ∈ x4.
The subproof is completed by applying ordsuccI2 with x2.
Claim L8: x2 ∈ x0
Apply nat_trans with
x0,
x1,
x2 leaving 3 subgoals.
The subproof is completed by applying H0.
The subproof is completed by applying H3.
The subproof is completed by applying L7.
Apply H1 with
x2,
λ x3 x4 . x3 ∈ Inj1 x0 leaving 2 subgoals.
The subproof is completed by applying L8.
Apply Inj1I2 with
x0,
x2.
The subproof is completed by applying L8.
Assume H3: x1 = x0.
Apply H3 with
λ x2 x3 . x3 ∈ Inj1 x0.
Apply nat_inv with
x0,
x0 ∈ Inj1 x0 leaving 3 subgoals.
The subproof is completed by applying H0.
Assume H4: x0 = 0.
Apply H4 with
λ x2 x3 . x3 ∈ Inj1 x0.
The subproof is completed by applying Inj1I1 with x0.
Apply exandE_i with
nat_p,
λ x2 . x0 = ordsucc x2,
x0 ∈ Inj1 x0 leaving 2 subgoals.
The subproof is completed by applying H4.
Let x2 of type ι be given.
Apply H6 with
λ x3 x4 . x4 ∈ Inj1 x0.
Claim L7: x2 ∈ x0
Apply H6 with
λ x3 x4 . x2 ∈ x4.
The subproof is completed by applying ordsuccI2 with x2.
Apply H1 with
x2,
λ x3 x4 . x3 ∈ Inj1 x0 leaving 2 subgoals.
The subproof is completed by applying L7.
Apply Inj1I2 with
x0,
x2.
The subproof is completed by applying L7.
Let x0 of type ι be given.
Apply Inj1_setsum_1L with
x0,
λ x1 x2 . x2 = ordsucc x0.
Apply L0 with
x0.
The subproof is completed by applying H1.