∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α]
  [inst : Inhabited α],
  t.WF → ∀ {he : Std.DTreeMap.Internal.Impl.contains t.minKey! t = true}, t.getKey t.minKey! he = t.minKey!