∀ {α : Type u_1} {β : Type u_2} {p : β → Prop} [inst : DecidablePred p] {f : α ≃ Subtype p} {g : Equiv.Perm α}
  {s t : Set α}, Set.SurjOn (⇑g) s t → Set.SurjOn (⇑(g.extendDomain f)) (Subtype.val ∘ ⇑f '' s) (Subtype.val ∘ ⇑f '' t)