Let x0 of type ο be given.
Let x1 of type ο be given.
Assume H0:
not (or x0 x1).
Apply andI with
not x0,
not x1 leaving 2 subgoals.
Assume H1: x0.
Apply H0.
Apply orIL with
x0,
x1.
The subproof is completed by applying H1.
Assume H1: x1.
Apply H0.
Apply orIR with
x0,
x1.
The subproof is completed by applying H1.