Let x0 of type ο be given.
Let x1 of type ι → ι be given.
Let x2 of type ι → ι be given.
Assume H0: x0.
Apply functional extensionality with
If_ii x0 x1 x2,
x1.
Let x3 of type ι be given.
Apply If_i_1 with
x0,
x1 x3,
x2 x3.
The subproof is completed by applying H0.