Let x0 of type ι be given.
Apply unknownprop_56bd0714abefd533b13603d171a24196c02fb0b6a0af8036287a8ec089f8957d with
λ x1 x2 : ι → ο . x2 x0 ⟶ ∀ x3 : ι → ο . (∀ x4 x5 . x3 (setsum x4 x5)) ⟶ x3 x0.
Let x1 of type ι → ο be given.
Assume H1:
∀ x2 x3 . x1 (setsum x2 x3).
Apply H0 with
λ x2 x3 . x1 x2.
The subproof is completed by applying H1 with
ap x0 0,
ap x0 1.