To create a PM4Py process tree for a hypothetical purchase-to-pay process, you would need to define the following operations:

- `A` - Start transaction.
- `B` - Execute `A`.
- `C` - End transaction.

The root node of this tree could be anything that has any children or is part of the process tree. The child nodes can be created by creating a ProcessTree object and calling its constructor with the appropriate operators and labels. For example:

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

# Create a ProcessTree object for A
root = ProcessTree()

# Create child processes B and C
child_processes_B = ProcessTree(label='B')
child_processes_C = ProcessTree(label='C')

# Add the child processes to root tree
for child_process in [child_processes_B, child_processes_C]:
    parent = root
    while parent.children:
        parent = parent.children[0]
```

Once you have defined this process tree, you can use the `ProcessTree` object to traverse and analyze the nodes. This process tree can be used to represent a purchase-to-pay process in PM4Py.

Note that the above code is not complete, it requires more detailed planning for the nodes (i.e., "start", "execute", "end" etc) and how they should connect. Also note that you will need to include your own implementation of the operators that are defined on this tree, as those can be complex operations in PM4Py.