feat: add factorial one-liner

@@ -197,3 +197,9 @@ seen = set(); unique = [x for x in items if not (x in seen or seen.add(x))]

merged = {item['id']: item for item in list_of_dicts}

```

**Example:** `[{'id': 1, 'name': 'A'}, {'id': 2, 'name': 'B'}] → {1: {'id': 1, 'name': 'A'}, 2: {'id': 2, 'name': 'B'}}`

## Calculate factorial of a number

```python

import math; fact = math.factorial(n)

```

**Example:** `n=5 → 120`