∀ {α : Type u} {β : α → Type v} [inst : Ord α] [Std.TransOrd α] [inst_2 : BEq α] [Std.LawfulBEqOrd α]
  {l : List ((a : α) × β a)},
  Std.Internal.List.DistinctKeys l →
    l.isEmpty = false → ∀ {fallback : α}, Std.Internal.List.containsKey (Std.Internal.List.maxKeyD l fallback) l = true