The function implemented by f(x) takes a string as input and returns the reverse of that string.

[CODE]:
```python
def reverse_string(input_string):
    return input_string[::-1]
```

This code uses slicing in Python to reverse the input string. By starting from the last element and going backwards with a step of -1, we can obtain the reverse of the input string.

Please note that this is just an approximation based on the experiments we have conducted so far. It is recommended to test the function with various other inputs to gain a more comprehensive understanding.