∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.ExtDTreeMap α β cmp} [inst : Std.TransCmp cmp]
  [inst_1 : Std.LawfulEqCmp cmp] {l : List ((a : α) × β a)} {k k' : α} (k_eq : cmp k k' = Ordering.eq) {v : β k}
  {fallback : β k'},
  List.Pairwise (fun a b => ¬cmp a.fst b.fst = Ordering.eq) l →
    ⟨k, v⟩ ∈ l → (t.insertMany l).getD k' fallback = cast ⋯ v