∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α],
  t.WF →
    t.isEmpty = false →
      ∀ {km fallback : α}, t.minKeyD fallback = km ↔ t.getKey? km = some km ∧ ∀ k ∈ t, (compare km k).isLE = true