In concilio Nicœni tractatus—“the Council of the Nicene tractate or creed,” possibly. The reference is plain, though there are various readings, and tractatus may not mean the creed. The real difficulty is that in the 20 extant Canons of Nicæa, there is no reference of the kind, and there is no evidence that any are missing. Perhaps St. Ambrose is quoting from memory, or some faulty collection, and so other canons are wrongly spoken of as Nicene. On the subject comp. St. Ambr. de Off. I. 257, and Dict. Chr. Ant. art. “Digamy.”