Here is an example of how you could implement a process tree for a Purchase-to-Pay process in pm4py:

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

# Define the activity labels
RECEIVE_ORDER = "Receive Order"
EDIT_ORDER = "Edit Order"
APPROVE_ORDER = "Approve Order"
RECEIVE_INVOICE = "Receive Invoice"
VALIDATE_INVOICE = "Validate Invoice"
PAY_INVOICE = "Pay Invoice"
CLOSE_PROCESS = "Close Process"

# Define the sequence of activities
def create_process_tree():
    global_order = ProcessTree(Operator.PARALLEL)
    
    # First step: Receive Order and Edit Order are executed in parallel
    receive_order = ProcessTree(label=RECEIVE_ORDER, parent=global_order)
    edit_order = ProcessTree(label=EDIT_ORDER, parent=global_order)
    
    # Append children to global order parallel operator
    global_order.children.append(receive_order)
    global_order.children.append(edit_order)
    
    # Option: Approve Order
    sequence_approve_order = ProcessTree(Operator.SEQUENCE, parent=global_order)
    
    # Approve Order
    approve_order = ProcessTree(label=APPROVE_ORDER, parent=sequence_approve_order)
    
    # Appends children to sequence approve operator
    sequence_approve_order.children.append(approve_order)
    
    # Option: Invoice handling
    sequence_invoice_handling = ProcessTree(Operator.SEQUENCE, parent=global_order)
    
    # Receive Invoice
    receive_invoice = ProcessTree(label=RECEIVE_INVOICE, parent=sequence_invoice_handling)
    validate_invoice = ProcessTree(label=VALIDATE_INVOICE, parent=sequence_invoice_handling)
    pay_invoice = ProcessTree(label=PAY_INVOICE, parent=sequence_invoice_handling)
    
    # Append children to sequence invoice operator
    sequence_invoice_handling.children.append(receive_invoice)
    sequence_invoice_handling.children.append(validate_invoice)
    sequence_invoice_handling.children.append(pay_invoice)
    
    # Closing step: Close Process
    close_process = ProcessTree(label=CLOSE_PROCESS, parent=global_order)
    
    # Append children to global order parallel operator
    global_order.children.append(sequence_approve_order)
    global_order.children.append(sequence_invoice_handling)
    global_order.children.append(close_process)

create_process_tree()
```

In this example, the Purchase-to-Pay process tree is created using the described operators. The Receive Order and Edit Order steps are the first steps in the