"A social system [...] is any system generated by the interaction of two or more behaving units. The basic criterion for establishing the existence of such a system is the existence of meaningful interdependence between the actions of the units (interaction). Thus[,] the consequences of actions by any one unit can be traced through the system; ultimately[,] these consequences 'feed back' to the...

1. Overview Notions of program equivalence are fundamental to the process of program speci-cation 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 ...