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) ⟶ setsum x3 (ap x2 1) = x2.
Apply proj1_ap_1 with
x2,
λ x3 x4 . x2 ∈ lam x0 (λ x5 . x1 x5) ⟶ setsum (proj0 x2) x3 = x2.
The subproof is completed by applying proj_Sigma_eta with x0, λ x3 . x1 x3, x2.