Pickup and Deliveries Constraint

[Sirine Belguith]

I'm solving the VRPTW problem with pickup and delivery.   Each vehicle is capable of doing delivery, pickup, or both at once.

     # Add Pickup and Delivery constraints.

for node_index in range(1, len(data['demands'])):

    if data['demands'][node_index] > 0 or data['deliveries'][node_index] > 0:

        pickup_index = manager.NodeToIndex(node_index)

        delivery_index = manager.NodeToIndex(node_index)

        # Add Pickup and Delivery constraints.

        routing.AddPickupAndDelivery(pickup_index, delivery_index)

        routing.solver().Add(routing.VehicleVar(pickup_index) == routing.VehicleVar(delivery_index))

        routing.solver().Add(time_dimension.CumulVar(pickup_index) <= time_dimension.CumulVar(delivery_index))

What is the constraint that ensures equal pickup and delivery for each trip and that every pickup has a delivery for every vehicle tour?