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