Expand single transition - sjdayday/xschema GitHub Wiki
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In addition to expanding the ongoing process, any other single transition can be expanded. In the case of Grasp, before we are ready to Close_hand, we must pre-shape the hand while approaching the object. We model this as the Prepare transition, beginning with Pre-shape:
Initially, Pre-shape is just the basic control flow. As it replaces the Prepare transition, we add a transition to either end of the basic control flow, and connect the arcs from the Enabled state and to the Ready state, to the corresponding transitions.
Next, we repeat the expand process for the Approach x-schema, with one difference. The default semantics of a single transition is that the x-schema(s) that replace the transition must all complete before the transition marks the outbound place. Instead of adding a new transition at the beginning and end of Approach, we add arcs to the same transitions that were added for Pre-shape, so both x-schemas begin from the same transition and end at the same transition. This implements "and" logic; the Prepare transition is complete when both Pre-shape and Approach are complete.
The implementation of expanding a single transition uses merge places to link the parent Petri net to each of the child Petri nets, just as was done for Grasp and Close_hand.
