Let x0 of type ι be given.
Let x1 of type ι → ι be given.
Let x2 of type ι be given.
Apply proj0_ap_0 with
x2,
λ x3 x4 . x2 ∈ lam x0 (λ x5 . x1 x5) ⟶ ap x2 1 ∈ x1 x3.
Apply proj1_ap_1 with
x2,
λ x3 x4 . x2 ∈ lam x0 (λ x5 . x1 x5) ⟶ x3 ∈ x1 (proj0 x2).
The subproof is completed by applying proj1_Sigma with x0, λ x3 . x1 x3, x2.