Flow lines in activity-charts are not modeled in the prototype. Flow lines signify that data from one activity are available to another activity; they do not model a specific channel through which the data flow. Therefore, though the data that flows through the flow lines is accessible to the Debugger, the flow lines themselves are not. The existence of flow lines in the activity-chart determines the externality of the element and whether it is available for use by the environment and primitive activities.