Let x0 of type ι be given.
Let x1 of type ι be given.
Let x2 of type ι be given.
Let x3 of type ι be given.
Apply binunionI1 with
SetAdjoin (UPair x0 x1) x2,
Sing x3,
x1.
The subproof is completed by applying unknownprop_91640ab91f642c55f5e5a7feb12af7896a6f3419531543b011f7b54a888153d1 with x0, x1, x2.