∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} [inst : Std.TransCmp cmp] [inst_1 : BEq α] [Std.LawfulBEqCmp cmp]
  {l : List (α × β)} {k : α} {fallback : β},
  (List.map Prod.fst l).contains k = false → (Std.ExtTreeMap.ofList l cmp).getD k fallback = fallback