Let x0 of type ι be given.
Let x1 of type ι be given.
Let x2 of type ι be given.
Assume H0: x2 ∈ x1.
Apply UnionI with
UPair x0 x1,
x2,
x1 leaving 2 subgoals.
The subproof is completed by applying H0.
The subproof is completed by applying UPairI2 with x0, x1.