Let x0 of type ι → ι → ο be given.
Let x1 of type ι be given.
Let x2 of type ι → ι → ο be given.
Assume H0: ∀ x3 x4 . x0 x3 x4 ⟶ x2 x3 x4.
Assume H1: ∀ x3 . x2 x3 x3.
Assume H2: ∀ x3 x4 . x2 x3 x4 ⟶ x2 x4 x3.
Assume H3: ∀ x3 x4 x5 . x2 x3 x4 ⟶ x2 x4 x5 ⟶ x2 x3 x5.
The subproof is completed by applying H1 with x1.