Let x0 of type ο be given.
Let x1 of type ο be given.
Let x2 of type ο be given.
Assume H1:
x0 ⟶ not x1 ⟶ x2.
Assume H2:
not x0 ⟶ x1 ⟶ x2.
Apply H0 with
x2 leaving 2 subgoals.
Apply H3 with
x2.
The subproof is completed by applying H1.
Apply H3 with
x2.
The subproof is completed by applying H2.