```python
purchase_to_pay_log_skeleton = {
    'equivalence': {
        ('Send_Purchase_Order', 'Receive_Purchase_Order_Confirmation'),
    },
    'always_before': {
        ('Receive_Invoice', 'Process_Invoice'),
        ('Approve_Payment', 'Make_Payment'),
    },
    'always_after': {
        ('Receive_Goods', ('Approve_Invoice', 'Reject_Invoice')),
    },
    'never_together': {
        ('Reject_Invoice', 'Approve_Payment'),
    },
    'activ_freq': {
        'Send_Purchase_Order': {0, 1},
        'Receive_Purchase_Order_Confirmation': {0, 1},
        'Receive_Goods': {1, 1},
        'Receive_Invoice': {1, 1},
        'Approve_Invoice': {0, 1},
        'Reject_Invoice': {0, 1},
        'Process_Invoice': {0, 1},
        'Approve_Payment': {0, 1},
        'Make_Payment': {0, 1},
    },
    'directly_follows': {
        ('Send_Purchase_Order', 'Receive_Purchase_Order_Confirmation'),
        ('Receive_Purchase_Order_Confirmation', 'Receive_Goods'),
        ('Receive_Goods', 'Receive_Invoice'),
        ('Receive_Invoice', 'Process_Invoice'),
        ('Process_Invoice', ('Approve_Invoice', 'Reject_Invoice')),
        ('Approve_Invoice', 'Approve_Payment'),
        ('Approve_Payment', 'Make_Payment'),
    }
}
```

**Explanation:**

* **equivalence:**  A purchase order and its confirmation are expected to occur together.
* **always_before:** Invoices must be processed before payment is approved or made.
* **always_after:** Goods receipt precedes invoice approval or rejection.
* **never_together:**  Rejecting an invoice prevents payment approval.
* **activ_freq:** 
    * A purchase order can be sent once or not at all.
    * Confirmation is expected once or not at all.
    * Goods, invoices, approvals, and payments are expected to occur once or not at all.
* **directly_follows:** This defines the sequential flow of activities in the process.


**Note:** This is a simplified example.  A real-world Purchase-to-Pay process might have many more activities and complex dependencies. 


