Let x0 of type ι be given.
Let x1 of type ι be given.
Assume H2: x1 ∈ x0.
Let x2 of type ι → ι be given.
Assume H3: ∀ x3 . x3 ∈ x1 ⟶ x2 x3 ∈ x0.
Apply H0 with
{x2 x3|x3 ∈ x1}.
Apply H1 with
x1,
x2 leaving 2 subgoals.
The subproof is completed by applying H2.
The subproof is completed by applying H3.