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 ο be given.
Assume H0: x2 ∈ x0 ⟶ x3.
Assume H1: x2 ∈ x1 ⟶ x3.
Apply binunionE with
x0,
x1,
x2,
x3 leaving 3 subgoals.
The subproof is completed by applying H2.
The subproof is completed by applying H0.
The subproof is completed by applying H1.