On a judgment for the plaintiff upon an issue of law, the plaintiff may proceed in the manner prescribed by Rule 55(a) and (b), M.R.Civ.P., upon the failure of the defendant to answer. If judgment is for the defendant upon an issue of law and the taking of an account or the proof of any factis necessary to enable the court to complete the judgment, a reference may be ordered, as provided in Rule 55(b).