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