Let x0 of type ι be given.
Let x1 of type ι → ι be given.
Let x2 of type ι be given.
Assume H0:
x2 ∈ lam x0 (λ x3 . x1 x3).
Apply and3E with
setsum (proj0 x2) (proj1 x2) = x2,
proj0 x2 ∈ x0,
proj1 x2 ∈ x1 (proj0 x2),
proj1 x2 ∈ x1 (proj0 x2) leaving 2 subgoals.
Apply Sigma_eta_proj0_proj1 with
x0,
x1,
x2.
The subproof is completed by applying H0.
Assume H2:
proj0 x2 ∈ x0.
The subproof is completed by applying H3.