[IDEM libro I quaestionum. ] §1.3.37.prSi de interpretatione legis quaeratur, in primis inspiciendum est, quo iure ciuitas retro in eiusmodi casibus usa fuisset: optima enim est legum interpres consuetudo.
[THE SAME, Questions, book 1] If a question arises regarding the interpretation of a law, first of all it must be examined what law the state had previously used in such cases; for custom is the best interpreter of laws.