Let x0 of type ο be given.
Let x1 of type ο be given.
Assume H0: x0 ⟶ x1.
Assume H1: x1 ⟶ x0.
Apply prop_ext with
x0,
x1.
Apply iffI with
x0,
x1 leaving 2 subgoals.
The subproof is completed by applying H0.
The subproof is completed by applying H1.