or agency is added as a defendant by amendment, the notice requirements of Rule 15(c)(1)(C)(i) and (ii) are satisfied if, during the stated period, process was delivered or mailed to the United States attorney or the United States attorney's designee, to the Attorney General of the United States, or to the officer or agency. (d) Supplemental Pleadings. On motion and reasonable notice, the court may, on just terms, permit a party to serve a supplemental pleading setting out any transaction, occurrence, or event that happened after the date of the pleading to be supplemented. The court may permit supplementation even though the original pleading is defective in stating a claim or defense. The court may order that the opposing party plead to the supplemental pleading within a specified time. Note to Subdivision (a) . The right to serve an amended pleading once as of course is common. 4 Mont.Rev.Codes Ann. (1935) 9186; 1 Ore.Code Ann. (1930) 1 -- 904; 1 S.C.Code (Michie, 1932) 493; English Rules Under the Judicature Act (The Annual Practice, 1937) O. 28, r. 2. Provision for amendment of pleading before trial, by leave of court, is in almost every code. If there is no statute the power of the court to grant leave is said to be inherent. Clark, Code Pleading , (1928) pp. 498, 509. Note to Subdivision (b) . Compare [former] Equity Rule 19 (Amendments Generally) and code provisions which allow an amendment "at any time in furtherance of justice," (e. g., Ark.Civ.Code (Crawford,