Huguccio, Summa decretorum to D.1 c.7, Distinctiones I-XX, ed. Oldřich Přerovský (Monumenta iuris canonici, Series A, 6. Città del Vaticano: Biblioteca Apostolica Vaticana, 2006) 42.

Adhering to the norm of moderamen inculpatae tutelae, means that if you are attacked with arms you may resist with arms. If you would be attacked by persons without arms, you cannot repel your attacker with arms without punishment. But can I not strike back with a lance, knife or sword a person who has struck with a staff, club, or stone? Does the definition of arms include stones and clubs?Is it not permitted to strike back with a larger or longer lance, knife or stone when one is struck with a smaller one? To this last question, I believe it is permitted. To the first question I think that it is permitted to strike with a staff, club or stone no matter what their size, and it is not permitted to strike back with a lance, knife, sword or other metal. This will be judged according to the decision of a judge and good men.

K. Pennington, "Moderamen inculpatae tutelae: The Jurisprudence of a Justifiable Defense," Rivista internazionale di diritto comune 24 (2013) 27-55.