Let x0 of type ι → ι → ι be given.
Let x1 of type ι be given.
Let x2 of type ι be given.
Let x3 of type ι be given.
Let x4 of type ι be given.
Assume H0: x1 = x2.
Assume H1: x3 = x4.
Apply H0 with
λ x5 x6 . x0 x6 x3 = x0 x2 x4.
Apply H1 with
λ x5 x6 . x0 x2 x6 = x0 x2 x4.
Let x5 of type ι → ι → ο be given.
Assume H2: x5 (x0 x2 x4) (x0 x2 x4).
The subproof is completed by applying H2.