The function shifts each letter backward in the alphabet by 2 positions, wrapping from a to z and A to Z. Digits and special characters remain unchanged.

[CODE]:
```python
def f(x: str) -> str:
    result = ""
    for char in x:
        if char.islower():
            result += chr(((ord(char) - ord("a") - 2) % 26) + ord("a"))
        elif char.isupper():
            result += chr(((ord(char) - ord("A") - 2) % 26) + ord("A"))
        else:
            result += char
    return result
```

Let's perform one more test to confirm this pattern:

COMMAND: PYTHON(./temp/function.py "Chemistry99!" "Computer")