 Sure! Here's an example of a POWL model for a Purchase-to-Pay process in Python, using the `pm4py` library:

```python
import pm4py
from pm4py.objects.powl.obj import StrictPartialOrder, OperatorPOWL, Transition, SilentTransition
from pm4py.objects.process_tree.obj import Operator

# Define the activities of the process
request_purchase = Transition(label="Request Purchase")
approve_purchase = Transition(label="Approve Purchase")
order = Transition(label="Order")
receive_invoice = Transition(label="Receive Invoice")
approve_payment = Transition(label="Approve Payment")
pay_invoice = Transition(label="Pay Invoice")

# Define the control-flow operators
purchase_loop = OperatorPOWL(operator=Operator.LOOP, children=[request_purchase, approve_purchase])
payment_loop = OperatorPOWL(operator=Operator.LOOP, children=[receive_invoice, approve_payment])

# Define the partial orders between nodes
request_order_order = StrictPartialOrder(nodes=[purchase_loop, order])
request_order_order.order.add_edge(purchase_loop, order)

approve_payment_pay = StrictPartialOrder(nodes=[payment_loop, pay_invoice])
approve_payment_pay.order.add_edge(payment_loop, pay_invoice)

# Combine the partial orders into the final POWL model
root = StrictPartialOrder(nodes=[request_order_order, approve_payment_pay])
root.order.add_edge(request_order_order, approve_payment_pay)
```

This POWL model represents a Purchase-to-Pay process that follows these steps:

1. The 'Request Purchase' activity is followed by the 'Approve Purchase' activity. This constitutes a loop that can be executed multiple times until the purchase is approved.
2. Once the purchase is approved, the 'Order' activity is executed.
3. The 'Receive Invoice' activity is followed by the 'Approve Payment