To remedy this evil, Justinian resolved upon the publication of a single treatise in which the commentaries and other writings of the jurists might be digested and harmonized. The preparation of this great work was intrusted to Tribonian, with the assistance of Theophilus, a celebrated professor of law at Berytus (modern Beyrout), and two other professors, and it was completed in the almost incredibly short period of four years. It was published in fifty books, under the title Digesta or Pandectae. While the Digest was in course of preparation Justinian resolved on the composition of a third work--viz., a systematic and elementary treatise on the law which might serve as a text-book for the use of students, and as an introduction to the larger work. The preparation of this was also intrusted to Tribonian and his colleagues, and having been completed a few days before the Digest, was published in four books on the same day (December 31, 534), under the title of Institutiones. It is based upon the Institutes of Gaius, and is familiar to all modern lawyers under the name of "Justinian's Institutes." Meantime, while both the Digest and the Institutes were being prepared, the Code of 529 above mentioned was withdrawn from circulation and republished in 534 with some alterations, and especially with the addition of fifty new constitutions (known as the Quinquaginta Decisiones) which had in the interim been pronounced by Justinian. This new edition, in twelve books, is known as the Codex Repetitiae Proelectionis, and is the one which has come down to us, no copy of the earlier codex being extant. All these works (Code, Digest, Institutes) were written originally in Latin, and all of them were prepared with care and skill, and testify to the great ability of Tribonian and his co-editors. Upon the publication of the "Digest" Justinian declared by a constitution that all previous law-books and decisions were to be held as superseded and it was forbidden to refer to them in the practice of the courts. During the subsequent years of his reign Justinian pronounced from time to time several new constitutions or laws, some of them making very important changes in certain departments of the law. These (mostly in Greek) were collected and published under the title of "Novellae" (_i.e._, "The Novels" or "New Works"). There were, so far as can be ascertained, about one hundred and seventy of these Novels. The Institutes, Digest, Code, and Novels together make up what is known as the Corpus Juris Civilis.