The Jiaoshi yilin 焦氏易林 "Master Jiao's forest of changes" is a book on diviniation with the help of hexagrams compiled by the Han period 漢 (206 BCE-220 CE) master Jiao Yanshou 焦延壽, courtesy name Jiao Gan 焦贛 (it is not sure whether Yanshou was his courtesy name, or Gan). He came from Liang 梁 (modern Shangqiu 商丘, Henan) and was a teacher in the household of the Prince of Liang. Later on he was appointed clerk (li 吏) in a commandery (jun 郡) and was then promoted to district magistrate (ling 令) of Xiaohuang 小黃 (near modern Kaifeng 開封, Henan). His teachers were famous Confucian professors like Meng Xi 孟喜 and Jing Fang 京房, both of them experts in the Confucian Classic Yijing 易經 "Book of Changes". Except the 16 juan "scrolls" long Yilin, Jiao Gan has also written the book Yilin bianzhan 易林變占, with the same length, yet this book went lost after the Tang period 唐 (618-907)
The Jiaoshi yilin explains that the 64 hexagrams are not only produced out of a single structure or basic hexagram (like qian 乾, Heaven), but are multiplied with each other to form 4,096 newly changed hexagrams. For each of these potential transformations, the author writes a short analysis consisting of a 4-syllable rhyme, an idea that has its origin in a sentence of the history Zuozhuan 左傳 (Fenghuang yu fei, he ming qiangqiang. 鳳凰于飛，和鳴鏘鏘 "The male and female phoenix fly together, singing harmoniously with gem-like sounds."). The book is specialized on the explanation of the hexagrams by the changes of single lines (guabian 卦變), the integration of cardinal directions, the Five Processes (wuxing 五行) and the Ten Celestial Stems (shi tiangan 十天干) into the interpretation of the trigrams (najia 納甲), as well as the evident or hidden character of particular hexagram lines (feifu 飛伏). These changes can be seen in the continuing increase and decrease of the agents Yin and Yang 陰陽, in natural disasters or personal luck and fortune.
The Yilin is an important early writing for a lot of divination schools in ancient China, especially for masters practicing divination by observing celestial phenomena (zhanhou 占候). Many Chinese scholars doubted that the Yilin was written by Jiao Gan. The imperial bibliography Yiwenzhi 藝文志 in the official dynastic history Hanshu 漢書, for instance, lists 13 books of experts on the Book of Changes, and 15 books on milfoil and turtle plastron divination, but a book of Jiao Gan is not included. In the imperial bibliography Jingjizhi 經籍志 in the history Suishu 隋書 the Jiaoshi yilin is included in the list of books on the Five Processes. The Qing period 清 (1644-1911) scholar Gu Yanwu 顧炎武 called the Jiaoshi yilin a book of the Later Han period, while Jiao Gan lived during the late Former Han period 前漢 (206 BCE-8 CE), when the Zuozhuan was not yet made a Classic. Yet the Yilin quotes a lot of sentences from the Zuozhuan, as well as sentences from the Hanshu, the history of the Former Han period that was only written in the early Later Han period 後漢 (25-220 CE). There is a more recent discussion on this topic, to be found in Qian Tongshu's 錢鐘書 book Guanzhuipian 管錐篇.
The Jiaoshi yilin in included in the collectanea Han-Wei congshu 漢魏叢書, Jindai mishu 津逮秘書, Siku quanshu 四庫全書 and Xuejin taoyuan 學津討原.
Source: Li Xueqin 李學勤, Lü Wenyu 呂文鬰 (1996). Siku da cidian 四庫大辭典, vol. 2, p. 1788. Changchun: Jilin daxue chubanshe.