Let x0 of type ο be given.
Let x1 of type ο be given.
Apply exactly1of2_E with
x0,
x1,
not x1 ⟶ x0 leaving 3 subgoals.
The subproof is completed by applying H0.
Assume H1: x0.
The subproof is completed by applying H1.
Assume H2: x1.
Apply H3 with
x0.
The subproof is completed by applying H2.