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