Here is the Python code to implement a process tree for an hypothetical Purchase-to-Pay process:
```
from pm4py.objects.process_tree.obj import ProcessTree, Operator

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

# Create the "Purchase" subtree
purchase = ProcessTree(operator=Operator.SEQUENCE, parent=root)
create_po = ProcessTree(label="Create Purchase Order", parent=purchase)
approve_po = ProcessTree(label="Approve Purchase Order", parent=purchase)
purchase.children.append(create_po)
purchase.children.append(approve_po)

# Create the "Order Goods" subtree
order_goods = ProcessTree(operator=Operator.SEQUENCE, parent=root)
create_pr = ProcessTree(label="Create Purchase Requisition", parent=order_goods)
receive_goods = ProcessTree(label="Receive Goods", parent=order_goods)
order_goods.children.append(create_pr)
order_goods.children.append(receive_goods)

# Create the "Pay Invoice" subtree
pay_invoice = ProcessTree(operator=Operator.SEQUENCE, parent=root)
create_inv = ProcessTree(label="Create Invoice", parent=pay_invoice)
pay_inv = ProcessTree(label="Pay Invoice", parent=pay_invoice)
pay_invoice.children.append(create_inv)
pay_invoice.children.append(pay_inv)

# Add the subtrees to the root node
root.children.append(purchase)
root.children.append(order_goods)
root.children.append(pay_invoice)
```
This process tree represents the following dependencies:

* The "Purchase" subtree consists of two sequential activities: "Create Purchase Order" and "Approve Purchase Order".
* The "Order Goods" subtree consists of two sequential activities: "Create Purchase Requisition" and "Receive Goods".
* The "Pay Invoice" subtree consists of two sequential activities: "Create Invoice" and "Pay Invoice".
* The three subtrees are executed in true concurrency, meaning that they can be executed in any order.

Note that this is just one possible representation of a Purchase-to-Pay process, and you may need to modify it to fit your specific requirements.