Let x0 of type ι be given.
Let x1 of type ι be given.
Assume H1: x1 ∈ x0.
Let x2 of type ι be given.
Assume H2: x2 ∈ x0.
Apply ZF_Union_closed with
x0,
UPair x1 x2 leaving 2 subgoals.
The subproof is completed by applying H0.
Apply ZF_UPair_closed with
x0,
x1,
x2 leaving 3 subgoals.
The subproof is completed by applying H0.
The subproof is completed by applying H1.
The subproof is completed by applying H2.