Let x0 of type ι be given.
Let x1 of type ι → ι be given.
Assume H0: ∀ x2 . x2 ∈ x0 ⟶ x1 x2 ∈ x0.
Let x2 of type ι → ο be given.
Assume H1:
∀ x3 . ∀ x4 : ι → ι . (∀ x5 . x5 ∈ x3 ⟶ x4 x5 ∈ x3) ⟶ x2 (pack_u x3 x4).
Apply H1 with
x0,
x1.
The subproof is completed by applying H0.