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,
x0.
The subproof is completed by applying unknownprop_2f981bb386e15ae80933d34ec7d4feaabeedc598a3b07fb73b422d0a88302c67 with x0, x1, x2.