Let x0 of type ι be given.
Let x1 of type (ι → ο) → ο be given.
Let x2 of type ι be given.
Assume H0: x2 ∈ x0.
Let x3 of type ι → ο be given.
Assume H1:
∀ x4 . ∀ x5 : (ι → ο) → ο . ∀ x6 . x6 ∈ x4 ⟶ x3 (pack_c_e x4 x5 x6).
Apply H1 with
x0,
x1,
x2.
The subproof is completed by applying H0.