Let x0 of type ι → ο be given.
Let x1 of type ι → ο be given.
Assume H0:
∀ x2 . iff (x0 x2) (x1 x2).
Apply functional extensionality with
x0,
x1.
Let x2 of type ι be given.
Apply prop_ext with
x0 x2,
x1 x2.
The subproof is completed by applying H0 with x2.