Let x0 of type ι be given.
Let x1 of type ι be given.
Apply UnionE_impred with
prim4 x0,
x1,
x1 ∈ x0 leaving 2 subgoals.
The subproof is completed by applying H0.
Let x2 of type ι be given.
Assume H1: x1 ∈ x2.
Assume H2:
x2 ∈ prim4 x0.
Apply PowerE with
x0,
x2,
x1 leaving 2 subgoals.
The subproof is completed by applying H2.
The subproof is completed by applying H1.