**Log Skeleton Model for Purchase-to-Pay Process**

```python
log_skeleton = {
    'equivalence': set(),
    'always_before': {
        ('Order', 'Invoice'),
        ('Invoice', 'Payment')
    },
    'always_after': {
        ('Request for Quote', 'Order'),
        ('Purchase Order', 'Invoice'),
        ('Goods Received', 'Invoice'),
        ('Invoice', 'Payment')
    },
    'never_together': {
        ('Order', 'Goods Received'),
        ('Purchase Order', 'Goods Received')
    },
    'activ_freq': {
        'Request for Quote': {0, 1},
        'Purchase Order': {1, 2},
        'Goods Received': {1, 2},
        'Invoice': {1, 2},
        'Payment': {1, 2}
    },
    'directly_follows': {
        ('Request for Quote', 'Purchase Order'),
        ('Purchase Order', 'Goods Received'),
        ('Goods Received', 'Invoice'),
        ('Invoice', 'Payment')
    }
}
```

**Constraints:**

* **Equivalence:** None.
* **Always Before:**
    * Order must occur before Invoice.
    * Invoice must occur before Payment.
* **Always After:**
    * Request for Quote must occur before Order.
    * Purchase Order must occur before Invoice.
    * Goods Received must occur before Invoice.
    * Invoice must occur before Payment.
* **Never Together:**
    * Order cannot coexist with Goods Received.
    * Purchase Order cannot coexist with Goods Received.
* **Activity Occurrences:**
    * Request for Quote can occur 0 or 1 times.
    * Purchase Order can occur 1 or 2 times.
    * Goods Received can occur 1 or 2 times.
    * Invoice can occur 1 or 2 times.
    * Payment can occur 1 or 2 times.
* **Directly-Follows:**
    * Request for Quote must directly follow Purchase Order.
    * Purchase Order must directly follow Goods Received.
    * Goods Received must directly follow Invoice.
    * Invoice must directly follow Payment.

**Note:**

* The activity 'Request for Quote' is optional and can occur only once per case.
* The activity 'Goods Received' can occur multiple times per case, but only after the corresponding Purchase Order.
* The activity 'Payment' must occur exactly once per case, following the 'Invoice' activity.