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