R13

R13 / 1:1c

Scenario is processing exactly one System Interaction

System Interaction is being processed by zero or one Scenario

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We process each System Interaction in sequence, one at a time. So at any given point in a running Scenario we know which step is the latest to be processed. Upon completion, we'll just retain the final interaction as current.

Any given System Interaction may or may not be the currently processed interaction.