∀ {α : Type u} {β : α → Type v} [inst : Ord α] [inst_1 : Std.LawfulEqOrd α]
  (mergeFn mergeFn_1 : (a : α) → β a → β a → β a),
  mergeFn = mergeFn_1 →
    ∀ (t₁ t₁_1 : Std.DTreeMap.Internal.Impl α β),
      t₁ = t₁_1 →
        ∀ (t₂ t₂_1 : Std.DTreeMap.Internal.Impl α β),
          t₂ = t₂_1 →
            Std.DTreeMap.Internal.Impl.mergeWith! mergeFn t₁ t₂ =
              Std.DTreeMap.Internal.Impl.mergeWith! mergeFn_1 t₁_1 t₂_1