Let x0 of type ι → ι → ο be given.
Apply H0 with
∀ x1 x2 . x0 x1 x2 ⟶ x0 x2 x2.
Let x1 of type ι be given.
Let x2 of type ι be given.
Assume H3: x0 x1 x2.
Claim L4: x0 x2 x1
Apply H1 with
x1,
x2.
The subproof is completed by applying H3.
Apply H2 with
x2,
x1,
x2 leaving 2 subgoals.
The subproof is completed by applying L4.
The subproof is completed by applying H3.