Abstract The Model Programming Language implements a form of data abstraction that has been used in a large programming project, the Demos Operating System. The use of the abstraction mechanism suggests a particular programming style that has evolved over an extensive period of gaining experience with the language. The programming style and Model's approach to data abstraction are both documented here using an example designed to illustrate several of the more important issues. The goal of the paper is to demonstrate a programming style and an approach to data abstraction in a programming language that has proved useful in a significant systems programming application.

Two of the more recently published systems for the efficient parsing of subclasses of deterministic, context-free languages are the backwards-deterministic, (or unambiguous, as they were originally called) simple precedence languages due to Wirth and Weber, and the reducing transition languages due to Eickel, Paul, Bauer, and Samelson. The main result demonstrated in this paper is that the backwards-deterministic, simple precedence languages are a proper subclass of the reducing transition languages. The reducing transition languages are certainly a subclass of the deterministic, context-free (LR(1)) languages but it is not known if this inclusion is proper.