  Interacting systems are increasingly common. Many examples pervade our
everyday lives: automobiles, aircraft, defense systems, telephone switching
systems, financial systems, national governments, and so on. Closer to computer
science, embedded systems and Systems of Systems are further examples of
interacting systems. Common to all of these is that some "whole" is made up of
constituent parts, and these parts interact with each other. By design, these
interactions are intentional, but it is the unintended interactions that are
problematic. The Systems of Systems literature uses the terms "constituent
systems" and "constituents" to refer to systems that interact with each other.
That practice is followed here. This paper presents a visual formalism, Swim
Lane Event-Driven Petri Nets, that is proposed as a basis for Model-Based
Testing (MBT) of interacting systems. In the absence of available tools, this
model can only support the offline form of Model-Based Testing.
