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,
x2.
The subproof is completed by applying unknownprop_ca66642b4e7ed479322d8970220318ddbb0c129adc66c35d9ce66f8223608389 with x0, x1, x2.