Let x0 of type ι be given.
Let x1 of type ι → ι be given.
Let x2 of type ι be given.
Let x3 of type ι be given.
Assume H0: x2 ∈ x0.
Assume H1: x3 ∈ x1 x2.
Apply UnionI with
prim5 x0 x1,
x3,
x1 x2 leaving 2 subgoals.
The subproof is completed by applying H1.
Apply ReplI with
x0,
x1,
x2.
The subproof is completed by applying H0.