∀ {α : Type u_2} {β : Type u_1} {f : α → β} {r : α → α → Bool} {s : β → β → Bool} {l l' : List α},
  (∀ a ∈ l, ∀ b ∈ l', r a b = s (f a) (f b)) → List.map f (l.merge l' r) = (List.map f l).merge (List.map f l') s