Let x0 of type ι be given.
Let x1 of type ι be given.
Let x2 of type ι be given.
Assume H0: x1 ∈ x2.
Apply V_eq with
x2,
λ x3 x4 . x0 ∈ x4.
Apply famunionI with
x2,
λ x3 . prim4 (V_ x3),
x1,
x0 leaving 2 subgoals.
The subproof is completed by applying H0.
Apply PowerI with
V_ x1,
x0.
The subproof is completed by applying H1.