∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {m₁ m₂ : Std.DTreeMap.Internal.Impl α β} [inst : Std.TransOrd α]
  (h₁ : m₁.WF) (h₂ : m₂.WF) {k : α} {h_contains : Std.DTreeMap.Internal.Impl.contains k (m₁.diff m₂ ⋯) = true},
  (m₁.diff m₂ ⋯).getKey k h_contains = m₁.getKey k ⋯