{α : Type u} →
  {β : α → Type v} →
    (k : α) →
      β k →
        (l r : Std.DTreeMap.Internal.Impl α β) →
          l.Balanced →
            r.Balanced → Std.DTreeMap.Internal.Impl.BalanceLErasePrecond l.size r.size → Std.DTreeMap.Internal.Impl α β