Literate Engines in Lisp

Civilization advances by extending the number of important operations which we can perform without thinking about them. { ALFRED NORTH WHITEHEAD (An introduction to Mathematics, 1911) An engine is a programming language abstraction that implements timed preemption. Engines form a useful mechanism where bounded computation is needed. We describe an implementation of engines in Common Lisp and illustrate its functionality. We conclude with a discussion on how engines are being used in an ongoing project about goal-driven explanation. This paper is also an exercise in writing a literate program in Lisp.