The function returns the input string unchanged, if it contains any digits or special characters. If the input string contains only alphabetical characters, it removes the second-to-last character.

[CODE]:
```python
def f(x):
    if any(char.isdigit() or not char.isalnum() for char in x):
        return x
    else:
        return x[:-2] + x[-1]
```

Let me know if you'd like me to confirm this with more tests, or if you have any questions.