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