Proofgold Proof

pf
Let x0 of type ι be given. Let x1 of type ι be given. Let x2 of type ι be given. Assume H0: nIn x0 x2. Assume H1: x2 = UPair x0 x1. Apply H0. Apply H1 with λ x3 x4 . x0 ∈ x4. The subproof is completed by applying UPairI1 with x0, x1. ■