Sure, here's an example Python code for implementing a process tree for a hypothetical Purchase-to-Pay process using the `pm4py` library:

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

# Define leaf nodes (activities)
place_order = ProcessTree(label="Place Order")
receive_goods = ProcessTree(label="Receive Goods")
create_invoice = ProcessTree(label="Create Invoice")
approve_invoice = ProcessTree(label="Approve Invoice")
make_payment = ProcessTree(label="Make Payment")

# Define sub-processes
order_process = ProcessTree(operator=Operator.SEQUENCE)
order_process.children.append(place_order)
order_process.children.append(receive_goods)

invoice_process = ProcessTree(operator=Operator.SEQUENCE)
invoice_process.children.append(create_invoice)
invoice_process.children.append(approve_invoice)

# Define the root process tree
purchase_to_pay = ProcessTree(operator=Operator.SEQUENCE)

# Set parent-child relationships
order_process.parent = purchase_to_pay
invoice_process.parent = purchase_to_pay
make_payment.parent = purchase_to_pay

purchase_to_pay.children.append(order_process)
purchase_to_pay.children.append(invoice_process)
purchase_to_pay.children.append(make_payment)

# Alternatively, you can use the constructor to set the parent-child relationships
# purchase_to_pay = ProcessTree(operator=Operator.SEQUENCE, children=[order_process, invoice_process, make_payment])
# order_process = ProcessTree(operator=Operator.SEQUENCE, children=[place_order, receive_goods], parent=purchase_to_pay)
# invoice_process = ProcessTree(operator=Operator.SEQUENCE, children=[create_invoice, approve_invoice], parent=purchase_to_pay)
# place_order.parent = order_process
# receive_goods.parent = order_process
# create_invoice.parent = invoice_process
# approve_invoice.parent = invoice_process
# make_payment.parent = purchase_to_pay
```

In this example, we first define the leaf nodes (activities) as individual `ProcessTree` objects with their respective labels. Then, we create sub-processes for the "Order" and "Invoice" phases by combining the relevant activities using the `Operator.SEQUENCE` operator.

Next, we create the root process tree `purchase_to_pay` with the `Operator.SEQUENCE` operator, indicating that the sub-processes should be executed in a specific order.

We set the parent-child relationships explicitly by assigning the parent to each child node and appending the child nodes to the parent's `children` list.

Alternatively, you can use the constructor to set the parent-child relationships by passing the child nodes as a list to the `children` parameter and setting the `parent` parameter for each child node.

This process tree represents the following workflow for the Purchase-to-Pay process:

1. Place Order
2. Receive Goods
3. Create Invoice
4. Approve Invoice
5. Make Payment

Note that you can modify the process tree structure and operators (`Operator.XOR`, `Operator.PARALLEL`, `Operator.LOOP`) based on the specific requirements of your Purchase-to-Pay process.