Let x0 of type ο be given.
Let x1 of type ο be given.
Assume H0:
and (x0 ⟶ x1) (x1 ⟶ x0).
Apply H0 with
iff x1 x0.
Assume H1: x0 ⟶ x1.
Assume H2: x1 ⟶ x0.
Apply iffI with
x1,
x0 leaving 2 subgoals.
The subproof is completed by applying H2.
The subproof is completed by applying H1.