Virtual reality (VR) and augmented reality (AR) systems are reaching sufficient maturity that some systems are being used on a regular basis by users and others are close to implementation. There are limitations, however, that have prevented many systems from being truly useful for users. An important limitation is the lack of understanding of human factors issues, such as human perceptual and cognitive limitations, that affect the potential utility of VR or AR systems.

Most abstract models of a set of parallel processes define a computation of the model to be a sequence. It is either a sequence of actions taken by the system [Lip] or a sequence of states of the system existing between actions [Kel, Lau, Ash]. Parallelsim is represented only by the fact that following a given action or state, the "next" action or state is not necessarily unique. That is, parallesim is represented by nondeterminism. A. W. Holt has called this representation approach "serializable concurrency" as opposed to "true concurrency" [Hol]. He, among others, has questioned the appropriateness of implying a total ordering between events that are only known to be partially ordered.In this paper, a definition of an algebraic model of a set of concurrently-executing sequential processes is presented. The "computations" of this model are directed acyclic graphs. The nodes of each computation graph represent computer operations and the edges represent the partial ordering of operations with respect to time.Examples demonstrate that these directed acyclic graphs aid in focusing a programmer/verifier's attention on the most important features of a computation. The model suggests the following verification paradigm: since systems often execute a certain sequence of actions to achieve a certain goal, verification procedures should identify these sequences, whether or not each is performed within a single process.A key notion in the abstract model is the treatment of synchronization mechanisms as full-fledged processes. A programming language syntax called "path programs" which is suggested by this notion is presented. Path programs are a generalization of the path expressions of Campbell and Habermann [CamHab]. The programming notation is based on the assumption that a processor can be dedicated to each shared data structure in the system. The duty of this processor is to control the synchronization of operations on the data structure.