**Process Tree Definition:**

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

# Create the root node of the process tree
root = ProcessTree(operator=Operator.PARALLEL)

# Create process trees for each stage of the Purchase-to-Pay process
purchase_order = ProcessTree(label="Purchase Order", parent=root)
goods_receipt = ProcessTree(label="Goods Receipt", parent=root)
invoice_approval = ProcessTree(label="Invoice Approval", parent=root)
payment = ProcessTree(label="Payment", parent=root)

# Create a sequence of steps within the Purchase Order stage
purchase_order_steps = ProcessTree(operator=Operator.SEQUENCE, parent=purchase_order)
purchase_order_steps.children.append(ProcessTree(label="Submit Purchase Order", parent=purchase_order_steps))
purchase_order_steps.children.append(ProcessTree(label="Review Purchase Order", parent=purchase_order_steps))
purchase_order_steps.children.append(ProcessTree(label="Approve Purchase Order", parent=purchase_order_steps))

# Create an exclusive choice between goods receipt and invoice approval
goods_receipt_or_invoice_approval = ProcessTree(operator=Operator.XOR, parent=goods_receipt)
goods_receipt_or_invoice_approval.children.append(ProcessTree(label="Goods Receipt", parent=goods_receipt_or_invoice_approval))
goods_receipt_or_invoice_approval.children.append(ProcessTree(label="Invoice Approval", parent=goods_receipt_or_invoice_approval))

# Create a loop for invoice approval steps
invoice_approval_loop = ProcessTree(operator=Operator.LOOP, parent=invoice_approval)
invoice_approval_loop.children.append(ProcessTree(label="Submit Invoice", parent=invoice_approval_loop))
invoice_approval_loop.children.append(ProcessTree(label="Review Invoice", parent=invoice_approval_loop))
invoice_approval_loop.children.append(ProcessTree(label="Approve Invoice", parent=invoice_approval_loop))

# Add the process trees as children to the root node
root.children.append(purchase_order)
root.children.append(goods_receipt)
root.children.append(invoice_approval)
root.children.append(payment)
```

**Notes:**

- The `label` attribute represents the name of each process tree node.
- The `operator` attribute specifies the type of operator used to connect process trees.
- The `children` list contains the child process trees.
- The `parent` attribute sets the parent process tree for each node.
- The loop operator is used to create a loop within the Invoice Approval stage. The loop continues until the "Approve Invoice" step is executed.

**Example Usage:**

```python
# Print the process tree diagram
print(root.get_graphviz_model())
```