∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} [Std.TransCmp cmp] [inst : BEq α] [Std.LawfulBEqCmp cmp]
  {l : List ((a : α) × β a)} {k : α}, (k ∈ Std.DTreeMap.ofList l cmp) = ((List.map Sigma.fst l).contains k = true)