The function takes a string input and returns a modified string. If the letter 'l' is found in the input, it replaces it with the letter 'q' only if the 'l' is in the lower case; else, it returns the same input string.

[CODE]:
```python
def approx_function(input_str):
    return input_str.replace('l', 'q') if 'l' in input_str else input_str
```
Keep in mind that this approximation could have some inaccuracies, but most of the function behavior is described with this approach.