Proofgold Proof

pf
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. Let x4 of type ι be given. Apply binunionI2 with SetAdjoin (SetAdjoin (UPair x0 x1) x2) x3, Sing x4, x4. The subproof is completed by applying SingI with x4. ■