Sure, below is an example of a simple Purchase-to-Pay process model implemented using Python's pm4py library.

```python
from pm4py.objects.process_tree.obj import ProcessTree, Operator

# Define the root node
root = ProcessTree(operator=Operator.PARALLEL)

# Define the purchase and payment activities
purchase = ProcessTree(label="Purchase")
payment = ProcessTree(label="Payment")

# Define the evaluate and execute activities
evaluate = ProcessTree(label="Evaluate")
execute = ProcessTree(label="Execute")

# Define the loop for the repeat activity
repeat = ProcessTree(operator=Operator.LOOP)

# Define the repeat activity
repeat_act = ProcessTree(label="Repeat")

# Add repeat activity as child and set as parent in the loop node
repeat_act.parent = repeat
repeat.children.append(repeat_act)

# Add purchase, evaluate, execute, payment activities in the repeat loop
repeat.children.extend([purchase, evaluate, execute, payment])

# Add the repeat loop as a child to the root node
root.children.append(repeat)

# Add purchase and payment activities as children to their respective parent nodes
evaluate.children.append(purchase)
payment.children.append(payment)

# Add evaluate and payment activities as children to the root node
root.children.extend([evaluate, payment])
```

In the code above, we first define the root node of the process tree using the `Operator.PARALLEL` operator. Then we define the four main activities of the Purchase-to-Pay process: `purchase`, `payment`, `evaluate` and `execute`. We also define a `repeat` activity and a loop using the `Operator.LOOP` operator. The `repeat` activity executes once and then, the loop starts. Inside the loop, the `purchase`, `evaluate`, `execute`, and `payment` activities are executed. The loop continues until the `payment` activity is executed. Finally, the `evaluate` and `payment` activities are directly executed after the loop.

Please note, this is a very basic model, and real-world processes can be much more complex. You might want to add more activities, split them into sub-processes, and so on.