Let x0 of type ι be given.
Let x1 of type ι → ι be given.
Let x2 of type ι be given.
Apply ReplEq with
x0,
x1,
x2,
x2 ∈ {x1 x3|x3 ∈ x0} ⟶ ∃ x3 . and (x3 ∈ x0) (x2 = x1 x3).
Assume H0:
x2 ∈ prim5 x0 x1 ⟶ ∃ x3 . and (x3 ∈ x0) (x2 = x1 x3).
Assume H1:
(∃ x3 . and (x3 ∈ x0) (x2 = x1 x3)) ⟶ x2 ∈ prim5 x0 x1.
The subproof is completed by applying H0.