The function takes a string as input and shifts each letter in the string forward by one in the alphabet. Numbers are left unchanged.

[CODE]:
```python
def f(x):
    output = ""
    for char in x:
        if char.isalpha():
            output += chr(ord(char) + 1)
        else:
            output += char
    return output
```

Please note that this is just an approximation based on the experiments. The actual implementation of the function may be more complex.