ions and Memories Ian Mason Stanford University IAM@SAIL.STANFORD.EDU Carolyn Talcott Stanford University CLT@SAIL.STANFORD.EDU 1. Overview Notions of program equivalence are fundamental to the process of program specification and transformation. Recent work of Talcott, Mason, and Felleisen establishes a basis for studying notions of program equivalence for programming languages with function and control abstractions operating on objects with memory. This work extends work of Landin, Reynolds, Morris and Plotkin. [Landin 1964] and [Reynolds 1972] describe high level abstract machines for defining language semantics. [Morris 1968] defines an extensional equivalence relation for the classical lambda calculus. [Plotkin 1975] extends these ideas to the call-by-value lambda calculus and defines the operational equivalence relation. Operational equivalence is the equivalence naturally associated with the operational approximation pre-ordering. One expression operationally approximates an...