Let x0 of type ι be given.
Let x1 of type ι be given.
Apply setminus_setminus with
x0,
x1,
x1,
λ x2 x3 . x0 = x2.
Apply setminus_selfannih with
x1,
λ x2 x3 . x0 = setminus x0 x3.
Let x2 of type ι → ι → ο be given.
The subproof is completed by applying setminus_idr with x0, λ x3 x4 . x2 x4 x3.