The function takes a string input and replaces the first character with the ASCII character shifted by a constant value. In this case, that constant value appears to be -7.

[CODE]: Here is a Python implementation that approximates the behavior of the function:

```python
def f(x):
    if len(x) == 0:
        return x
    else:
        return chr(ord(x[0]) - 7) + x[1:]
```
Keep in mind that the knowledge of the behaviour comes from limited results and there might be cases where the function returns different output. Make sure to consider additional test cases if needed.