Here is an example of Python code that implements a POWL model for an hypothetical Purchase-to-Pay process:
```
import pm4py
from pm4py.objects.powl.obj import StrictPartialOrder, OperatorPOWL, Transition, SilentTransition
from pm4py.objects.process_tree.obj import Operator

# Define transitions
Create_Purchase_Requisition = Transition(label="Create Purchase Requisition")
Approve_Purchase_Requisition = Transition(label="Approve Purchase Requisition")
Create_Purchase_Order = Transition(label="Create Purchase Order")
Send_Purchase_Order = Transition(label="Send Purchase Order")
Receive_Goods = Transition(label="Receive Goods")
Create_Invoice = Transition(label="Create Invoice")
Approve_Invoice = Transition(label="Approve Invoice")
Pay_Invoice = Transition(label="Pay Invoice")
Skip = SilentTransition()

# Define choice and loop nodes
Approve_PR_OR_Create_PO = OperatorPOWL(operator=Operator.XOR, children=[Approve_Purchase_Requisition, Create_Purchase_Order])
 Receive_Goods_Loop = OperatorPOWL(operator=Operator.LOOP, children=[Receive_Goods, Create_Invoice])

# Define partial order
PO = StrictPartialOrder(nodes=[
    Create_Purchase_Requisition,
    Approve_PR_OR_Create_PO,
    Create_Purchase_Order,
    Send_Purchase_Order,
    Receive_Goods_Loop,
    Approve_Invoice,
    Pay_Invoice
])
PO.order.add_edge(Create_Purchase_Requisition, Approve_PR_OR_Create_PO)
PO.order.add_edge(Approve_PR_OR_Create_PO, Create_Purchase_Order)
PO.order.add_edge(Create_Purchase_Order, Send_Purchase_Order)
PO.order.add_edge(Send_Purchase_Order, Receive_Goods_Loop)
PO.order.add_edge(Receive_Goods_Loop, Approve_Invoice)
PO.order.add_edge(Approve_Invoice, Pay_Invoice)
```
This POWL model represents a simplified Purchase-to-Pay process, with the following activities:

1. Create Purchase Requisition
2. Approve Purchase Requisition (or create Purchase Order directly)
3. Create Purchase Order
4. Send Purchase Order
5. Receive Goods (loop until all goods are received)
6. Create Invoice
7. Approve Invoice
8. Pay Invoice

The model includes two control-flow operators:

* An exclusive choice (XOR) between approving the purchase requisition and creating a purchase order
* A loop (LOOP) for receiving goods, which repeats until all goods are received

The partial order is defined