Let x0 of type ι be given.
Let x1 of type ι be given.
Let x2 of type ι be given.
The subproof is completed by applying and4I with
subfield x1 x0,
Field_Hom x0 x0 x2,
∀ x3 . x3 ∈ ap x0 0 ⟶ ∃ x4 . and (x4 ∈ ap x0 0) (ap x2 x4 = x3),
∀ x3 . x3 ∈ ap x1 0 ⟶ ap x2 x3 = x3.