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