That is that He commanded a high priest to marry a virgin. And that is His saying, "He shall marry a woman who is a virgin." And they said in the explanation (Ketuvot 30a), "Rabbi Akiva would designate as a mamzer even [a child resulting from the union of] those prohibited by a positive commandment." And the explanation of this is that it is when a high priest would have sexual intercourse with someone who is not a virgin - about which he is prohibited by a positive commandment. For the principle with us is that a negative commandment that is derived from a positive commandment is [also] a positive commandment. And behold the regulations of this commandment have already been explained in Yevamot, in Kiddushin and in Ketuvot. (See Parashat Emor; Mishneh Torah, Forbidden Intercourse 17.)