Sure! Assuming that the Purchase-to-Pay process involves the following steps:

1. Raise Purchase Order (PO)
2. Approve PO
3. Receive Invoice
4. Approve Invoice
5. Make Payment
6. Archive

And there is a possibility of rejection at any of the approval stages, let's create a POWL model to represent this process. We will use the following operators and labels for the transitions:

- Transitions: Raise PO, Approve PO, Reject PO, Receive Invoice, Approve Invoice, Reject Invoice, Make Payment, Archive
- Operators: Loop, XOR

Now, let's create the Python code to implement this POWL model using pm4py:

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

# Define transitions
raise_po = Transition(label="Raise PO")
approve_po = Transition(label="Approve PO")
reject_po = Transition(label="Reject PO")
receive_invoice = Transition(label="Receive Invoice")
approve_invoice = Transition(label="Approve Invoice")
reject_invoice = Transition(label="Reject Invoice")
make_payment = Transition(label="Make Payment")
archive = Transition(label="Archive")
skip = SilentTransition()

# Define loops
raise_po_loop = OperatorPOWL(operator=Operator.LOOP, children=[raise_po])
approve_po_loop = OperatorPOWL(operator=Operator.LOOP, children=[approve_po, reject_po])
receive_invoice_loop = OperatorPOWL(operator=Operator.LOOP, children=[receive_invoice])
approve_invoice_loop = OperatorPOWL(operator=Operator.LOOP, children=[approve_invoice, reject_invoice])
make_payment_loop = OperatorPOWL(operator=Operator.LOOP, children=[make_payment])

# Define XOR
approve_po_xor = OperatorPOWL(operator=Operator.XOR, children=[approve_po_loop, skip])
approve_invoice_xor = OperatorPOWL(operator=Operator.XOR, children=[approve_invoice_loop, skip])

# Define strict partial order
root = StrictPartialOrder(nodes=[
    raise_po_loop,
    approve_po_xor,
    receive_invoice_loop,
    approve_invoice_xor,
    make_payment_loop,
    archive
])

# Define order
root.order.add_edge(raise_po_loop, approve_po_xor)
root.order.add_edge(approve_po_xor, receive_invoice_loop)
root.order.add_edge(receive_invoice_loop, approve_invoice_xor)
root.order.add_edge(approve_invoice_xor, make_payment_loop)
root.order.add_edge(make_payment_loop, archive)
```

This POWL model represents the Purchase-to-Pay process with loops for raising POs, approving POs, receiving invoices, approving invoices, and making payments. It also includes the possibility of rejecting POs and invoices using XOR operators. The strict partial order ensures that the process flows in the correct sequence.