The image shows a Start step that has an outgoing control flow to a Pass-through step. The Pass-through step has an outgoing data flow going to a role port on the left side of the role (PO event) and an incoming data flow coming from a role port on the right side of the role. The Pass-through step has two outgoing control flows, each to separate Pass-through steps (one on the left and one on the right). Of these two Pass-through steps, the Pass-through step on the left has an incoming data flow from a role port on the left side of the role (ack event), an outgoing control flow to an Or step, and an outgoing data flow to a role port on the right side of the role. The Or step has an outgoing control flow to an End step. The Pass-through step on the right has an outgoing control flow to an Or step, an outgoing data flow to a role port on the right side of the role, and an incoming data flow from a role port on the left side of the role (nack event).