Let x0 of type ι be given.
Let x1 of type ι be given.
Apply pair_ap_0 with
x0,
x1,
λ x2 x3 . setsum x3 (ap (setsum x0 x1) 1) = setsum x0 x1.
Apply pair_ap_1 with
x0,
x1,
λ x2 x3 . setsum x0 x3 = setsum x0 x1.
Let x2 of type ι → ι → ο be given.
The subproof is completed by applying H0.