Certainly! Let's first describe a simple hypothetical Purchase-to-Pay process, then we can write the necessary code using the pm4py library to model this in a process tree.

### Hypothetical Purchase-to-Pay Process Description:
1. **Purchase Order Creation**: A purchase order (PO) is created either for services or for goods. This represents an exclusive choice.
2. **Approval**: Each PO must be approved.
3. **Order Fulfillment**: Once approved, if it is an order for goods, it involves two parallel processes - shipping and invoicing. If it is for services, it only involves invoicing.
4. **Payment**: After fulfillment, the final step is the payment.

Now, let's construct the process tree:

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

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

# Step 1: Purchase Order Creation (Exclusive choice between goods and services)
po_creation = ProcessTree(operator=Operator.XOR, parent=root)
goods_PO = ProcessTree(label="Create Goods PO", parent=po_creation)
services_PO = ProcessTree(label="Create Services PO", parent=po_creation)
po_creation.children.extend([goods_PO, services_PO])

# Step 2: Approval
approval = ProcessTree(label="Approve PO", parent=root)

# Step 3: Order Fulfillment (Exclusive choice on fulfillment based on the type of PO)
fulfillment = ProcessTree(operator=Operator.XOR, parent=root)

# Sub-steps for Goods (parallel processing for shipping and invoicing)
goods_fulfillment = ProcessTree(operator=Operator.PARALLEL, parent=fulfillment)
shipping = ProcessTree(label="Shipping", parent=goods_fulfillment)
goods_invoice = ProcessTree(label="Invoice for Goods", parent=goods_fulfillment)
goods_fulfillment.children.extend([shipping, goods_invoice])

# Sub-steps for Services (Just invoicing)
services_invoice = ProcessTree(label="Invoice for Services", parent=fulfillment)

fulfillment.children.extend([goods_fulfillment, services_invoice])

# Step 4: Payment
payment = ProcessTree(label="Make Payment", parent=root)

# Append all the major steps to the root
root.children.extend([po_creation, approval, fulfillment, payment])

# Now root is the root of the full Purchase-to-Pay process tree
```

This code builds a process tree for a simplified Purchase-to-Pay process with pm4py modulo some simplifications in handling the different paths for goods and services beyond the order creation phase. Each node in the tree represents a step in the process, with decision points, parallel processes, and sequences modeled accordingly.