YAILS is implemented using YAP-Prolog [Damas et.al,1989]. YAP is a very efficient implementation of Prolog. The main reason for this choice was a personal preference on Prolog. The always claimed problems of efficiency in comparison to languages like C were expected to be overcame not only by the speed of YAP but also by the availability of a powerful machine in our lab. On the other hand the expressiveness of Prolog enabled the rapid prototyping as well as high speed and conciseness on implementing YAILS. Finaly YAP-prolog has a portable graphical package [Leal,1992] which enables the construction of X-windows interfaces thus enabling the implementation of a nice user-interface for YAILS.