In 1969 Bell Labs employees including Thompson, Ritchie, Kernighan, McIlroy, Lesk and Ossanna wrote the Unix operating system in assembly language. By 1973 it had been recoded in 'C' by Kernighan and Ritchie. Lions' Commentary was the Unix kernel documentation approved for use outside Bell Labs. The 6th Edition Unix source code was permitted for classroom use, but when Unix Version 7 was announced in 1979, the license for classroom use of the book was withdrawn. Thousands of computer science students then made and reviewed photocopies, meeting after hours to discuss the commentary. Duplicates spread across the world.