The function simply returns the input string if it is not equal to "123", and returns an empty string if the input is "123".

[CODE]:
```python
def f(x):
    if x == "123":
        return ""
    else:
        return x
```

Please note that this is an approximation based on the observed output. There may be additional operations or conditions that the function performs, but further testing is required to determine those.