Apply functional extensionality with
setsum,
λ x0 x1 . lam 2 (λ x2 . If_i (x2 = 0) x0 x1).
Let x0 of type ι be given.
Apply functional extensionality with
setsum x0,
(λ x1 x2 . lam 2 (λ x3 . If_i (x3 = 0) x1 x2)) x0.
Let x1 of type ι be given.
The subproof is completed by applying tuple_pair with x0, x1.