Rule 13 F.R.Civ.P. applies in adversary proceedings, except that a party sued by a trustee or debtor in possession need not state as a counterclaim any claim that the party has against the debtor, the debtor’s property, or the estate, unless the claim arose after the entry of an order for relief. A trustee or debtor in possession who fails to plead a counterclaim through oversight, inadvertence, or excusable neglect, or when justice so requires, may by leave of court amend the pleading, or commence a new adversary proceeding or separate action.